Evidence suggests that differences in fatty acid composition among various fish species may be due to differences in diet or to environmental factors such as temperature, salinity, and depth at which the fish are caught. The beneficial effects of a diet containing fish on cardiovascular or other diseases have been associated with their high content of eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. In this study we analyzed the fatty acid composition of the flesh of 18 different species of marine fish and of cultured rainbow trout. The fish were obtained from the Black and the Marmara Seas, both of which have unique biological and ecological systems as well as eutrophication and pollution. The contents of 20:5n-3 and 22:6n-3 in the marine fish ranged from 4.2 to 13.3 wt% of total fatty acids, and from 6.6 to 40.8 wt%, respectively. The most important differences from other studies on oceanic fish were the tendencies toward higher percentages of 16:0 and 22:6n-3. The n-3 series of polyunsaturated fatty acids were present as 32.4 +/- 1.9% of the total fatty acids. The present study suggests that mature and immature Pomatomus saltator as well as Engraulis encrasicolus, Mullus surmuletus, Sardina pilchardus, Mugil cephalus, and Sarda sarda may be preferred for the Turkish diet as a result of their high 20:5 n-3 and 22:6 n-3 contents. The cultured rainbow trout Oncorhynchus mykiss is not as good a source of n-3 fatty acids as are the marine fish.