Nine species of the genus Propionibacterium were studied for in vitro biotransformation of unsaturated long chain fatty acids. P. acnes isomerized the 9-cis-double bond of linoleic acid into a 10-trans-double bond followed by partial reduction of the 12-cis-double bond. This species also isomerized the 9-cis-double bond of .gamma.-linolenic acid, the 10-cis-double bond of 10-cis, 13-cis-nonadecadienoic acid and the 12-cis-double bond of homo-.gamma.-linolenic acid and arachidonic acid. P. freudenreichii subsp. freudenreichii, P. freudenreichii subsp. shermanii, P. acidi-propionici and P. technicum isomerized the 12-cis-double bond of linoleic acid into an 11-trans-double bond partially followed by reduction of the 9-cis-double bond. These species also isomerized the 12-cis-double bond of linolenic acid and .gamma.-linolenic acid, P. granulosum, P. avidum, P. jensenii, P. thoenii and P. lymphophilum did not isomerize unsaturated long chain fatty acids. After prolonged incubation, all strains partially catabolized the long chain fatty acids in the culture medium.