S u m m r y . Several strains of Pseudomonas, Nocurdia and incompletely identified soil isolates have been grown in a mineral salts plus hydrocarbon medium, and the fatty acids produced by the organisms have been isolated, identified and estimated. The results of an estimation of the percentage conversion to these acids under varying experimental conditions is discussed in relation to the metabolic systems involved. Some indication has been obtained that the hydrocarbon breakdown pathway by these organisms is that of w oxidation followed by / 3 oxidation. Preliminary experiments carried out with one strain of organism, Pseudomonas aeruginosa 5940, indicate that there may be some difference in the utilization of odd-and evenchain hydrocarbons by this organism. An improvement of 3-8-fold was obtained by using nitrate instead of ammonium nitrogen ; 13-fold by using continuous instead of batch operation; and 8-fold by the use of liquid instead of solid paraffins, giving a total improvement of yield of 400-fold.