The fatty acid composition of the tunicate Botryllus schlosseri and of two bacterial strains found within the tunicate, namely Vibrio parahaemolyticus and of an associated but previously unreported gram positive cocci were studied. The polyunsaturated fatty acids 6,9,12-octadecatrienoic acid, 5,8,11,14,17-eicosapentaenoic acid, and 4,7,10,13,16,19-docosahexaenoic acid were particularly abundant in B. schlosseri and were not detected in the two bacterial strains found in the tunicate. The iso/anteiso pair, 13-methyltetradecanoic acid and 12-methyltetradecanoic acid, were the principal fatty acids in the gram positive cocci, and the 9- and 11-hexadecenoic acids were particularly abundant in V. parahaemolyticus. The diunsaturated fatty acid 9,12-octadecadienoic acid was also shown to be present in V. parahaemolyticus. The fatty acid composition of a third bacterial strain, characterized as either a Pseudomonas or an Alteromonas species, and shown to be present only in the sea water from the Black Sea and not in B. schlosseri, is also reported. This is the first investigation on fatty acids from Black Sea bacteria.