Hydrocarbon-assimilating yeasts and bacteria were isolated from soil and sewage. The optimal conditions of cell yield from liquid paraffine by a Torulopsis yeast and a Pseudomonas strain were studied. A Torulopsis yeast gave, in optimal condition, 70 percent cell yield on u weight conversion basis from light oil fraction. In a strain of Pseudomonas the additions of amino acids, Fe++, Mg++ and Ca++ ions were effective for cell production. This strain showed, in optimal condition, 80 percent cell yield (wt%) from kerosene.