Mucor rouxianus, Mucor spinescenes and Penicillium citrinum were used for this study．By these filamentous fungi long-chain fatty acids were utilized as sole cardon source. The short−chain fatty acidis had a little meaning as sole carbon source for these organisms. But formic acid，n-butyric acid and n-valeric acid inhibited the growth of these organisms. Among the long-chain fatty acids，the lauric acid only inhibited the growth of P. citrinum. This growth inhibition by fatty acids is due to the inhibition on the respiratory system. M. rouxianus formed so-called yeast-like cells in YP medium under aerobic condition. This phenomenon disappeared by exogenous addition of glucose or adequate carbon source. But，by the addition of valeric acid the formation of the yeast-like cells could not be discontinued． The growth inhibition by valeric acid was more conspicuously on n-valeric acid than iso-valeric acid.