Malassezia pachydermatis is one of the many infective agents involved in canine otitis externa. Two major phenotypes (large and small colony types), which had different fatty acid compositions, were found among strains of M pachydermatis isolated from canine ears or skin. The large-colony type contained palmitic, stearic, oleic and linoleic acids, whereas the small-colony type contained myristic, palmitic, stearic, linoleic acids and five other unidentified fatty acids. Exogenous saturated fatty acids in the medium had no effect on the growth of a large colony strain but oleic and linoleic acids were mycostatic. Adding fatty acids to the medium changed the fatty acid composition of the strain. Saturated fatty acids were incorporated as such into the cell, but the unsaturated fatty acids caused complex changes in composition, including the appearance of a new fatty acid. It is suggested that these changes in the fatty acid composition of M pachydermatis could explain the mycostatic effect of unsaturated fatty acids like oleic and linoleic acids at least in vitro.