In the cultivation of a micro-organism, particularly yeast, capable of growing on a feedstock consisting wholly or in part of straight-chain hydrocarbons, the micro-organism is cultivated in the presence of the feedstock, an aqueous nutrient medium and a gas containing free oxygen, and the feedstock is present in the fermenter in which cultivation takes place in the form of small particles. The small particles which are to be of an average diameter less than 30 microns, preferably less than 10 microns, may be liquid or solid. A dispersion of small particles in a liquid carrier medium is passed into the fermenter or small liquid particles are formed within the fermenter by direct atomization into the liquid material in the fermenter. The liquid medium may consist of aqueous nutrient medium or a hydrocarbon not metabolizable by the particular strain of micro-organism included in the feed stream, e.g. a de-waxed gas oil. The feedstock may include straight-chain C10 or higher paraffinic and/or olefinic hydrocarbons, kerosine, gas oils and lubricating oils, either unrefined or after some refinery treatment. When a petroleum fraction containing 3-45% of straight-chain hydrocarbons of at least 10 carbon atoms per molecule or a wax-containing petroleum gas oil is employed as the feedstock, there is obtained by the cultivation of the micro-organism a petroleum fraction having a reduced proportion of or no straight chain hydrocarbons, or a gas oil of reduced content of wax (as the case may be). Yeasts mentioned are Candida lipolytica, Candida pulcherrima, Candida utilis, Candida tropicalis, Torulopsis collisculosa, Hansenula anomala, Oidium lactis and Neurospora sitophila. The following micro organisms are also mentioned: Bacillus megatherium, Bacillus subtilis, Pseudomonas aeruginosa, Bacillus amylebacter, Pseudomonas natriegens, Arthrobacter sp., Micrococcus sp., Corynebacterium michiganense, Pseudomonas syringae, Xanthomonas begoniae, Flavobacterium sp. devorans, Acetobacter sp., Actinomyces sp., Agrobacterium sp., Aplanobacter sp., Penicillium expansum, Penicillium roqueforti, Penicillium notatum, Aspergillus fussigatus, Aspergillus niger and Aspergillus versicolor. In examples, substrates of gas oil and of paraffin wax are used.ALSO:In the cultivation of a micro-organism, particularly yeast, capable of growing on a feedstock consisting wholly or in part of straight chain hydrocarbons, the micro-organism is cultivated in the presence of the feedstock, an aqueous nutrient medium and a gas containing free oxygen, and the feedstock is present in the fermenter in which cultivation takes place in the form of small particles. The small particles which are to be of an average diameter less than 30 microns, preferably less than 10 microns, may be liquid or solid. A dispersion of small particles in a liquid carrier medium is passed into the fermenter or small liquid particles are formed within the fermenter by direct atomization into the liquid material in the fermenter. In the case of liquid small particles, these may be formed at atomizing e.g. under the action of ultra-sonic waves, a liquid phase feedstock into an aqueous phase or a gas phase, which latter is then transferred into a liquid medium. Small solid particles may be formed as a suspension in a liquid medium by atomizing melted feedstock, in the presence or absence of other constituents, into a liquid medium which is at a temperature below the melting point of the feedstock, whereby shock cooling of the feedstock takes place. Alternatively a liquid-in-liquid solution or suspension is formed by atomizing melted solid feedstock into a liquid medium which is at a temperature above the melting point of the feedstock, and subjecting this solution or suspension to shock cooling to form a solid-in-liquid suspension of the feedstock in the liquid medium. In a preferred procedure there is formed a dispersion of a feedstock in a liquid medium which either contains a micro-organism or with which a micro-organism is then mixed, and the resulting mixture is passed to the fermenter. The micro-organism present in the feed stream need not be of the same strain as that being cultivated in the fermenter. Its presence with the dispersed feedstock reduces the tendency of the particles to agglomerate. The liquid medium may consist of aqueous nutrient medium or a hydrocarbon not metabolizable by the particular strain of micro-organism included in the feed stream e.g. a dewaxed gas oil. The feedstock may include straight chain C10 or higher paraffinic and/or olefinic hydrocarbons, kerosine, gas oils and lubricating oils, either unrefined or after some refinery treatment. When a petroleum fraction containing 3-45% of straight chain hydrocarbons of at least 10 carbon atoms per molecule or a wax-containing petroleum gas oil is employed as the feedstock, there is obtained by the cultivation of the micro-organism a petroleum fraction having a reduced proportion of or no straight chain hydrocarbons, or a gas oil of reduced content of wax (as the case may be). Yeasts mentioned are Candida lipolytica, Candida pulcherrima, Candida utilis, Candida tropicalis, Torulopsis collisculosa, Hansenula anomala, Oidium lactis and Neurospora sitophila. The following micro-organisms are also mentioned: Bacillus megatherium, Bacillus subtilis, Pseudomonas aeruginosa, Bacillus amylebacter, Pseudomonas natriegens, Arthrobacter sp., Micrococcus sp., Corynebacterium michiganense, Pseudomonas syringae, Xanthomonas begoniae, Flavobacterium sp. devorans, Acetobacter sp., Actinomyces sp., Agrobacterium sp., Aplanobacter sp., Pencillium expansum, Pencillium roqueforti, Pencillium notatum, Aspergillus fussigatus, Aspergillus niger and Aspergillus versicolor. According to an Example (1) gas oil and culture medium were pumped at a back pressure of 14-16 Kg./cm2. through buffer reservoirs into a stainless steel capillary tube (interior diameter 0.5mm) having the free end narrowed to leave an outlet hole of 0.05-0.08mm. The mixture of gas-oil and culture medium from the injector passed into a five-litre continuous fermenter provided with an agitator, a ring of capillary air injectors, electrodes for automatic pH control, double external cladding to keep the temperature constant at 30 DEG C. by water circulation and a system for drawing off liquid by means of a piston pump. For the injection the capillary point of the injector was mounted either in the body of liquid preferably beneath the agitator, or above the surface and pointing towards the bottom of the fermenter. The cultivation of Candida lipolytica employing Iraq heavy gas-oil and a specified aqueous mineral nutrient medium is disclosed. In further Examples (2) molten hard gatsch and/or solid paraffin wax was injected as a fine dispersion in air into the culture medium in the fermenter, using an air atomizer consisting of a capillary tube mounted in a concentric air inlet tube; (3) gas-oil feedstock and the culture medium were pumped by a rotary pump to an ultra-sonic vibration apparatus known as a "fluid whistle" made up of a very fine steel plate mounted in front of a flattened tube which constituted the flat jet by which the liquids enter. The assembly was mounted in a cylindrical container forming a resonance chamber, the frequency employed was of the order of 20-22 kc./s and the outlet of the whistle was connected to the fermenter by means of a flexible plastic pipe of 50 cms length; (4) dispersion of a gas-oil feedstock in a mineral medium was carried out in a colloid mill in which the shearing effect was supplied by a toothed wheel and two conical surfaces all mounted on the same axis of rotation. In order to stabilize the emulsion formed, a quantity of culture liquor-equal to about 5% of the volume of the emulsion and containing 8-9 grams/litre of yeast was injected into the liquid material at either the inlet or the outlet of the colloid mill.