The total tetracosenoic acid (24∶1) levels in nine marine oils examined ranged from 0.4 to 1.1% of the total fatty acids. Gas-liquid chromatography (GLC) analysis of whole fish oil fatty acid methyl esters usually shows a single 24∶1 peak, as the dominant (60–90%) isomer is 24∶1ω9, and the minor peaks are not seen. Isolation and oxidative fission demonstrate that the lesser isomers present in these oils (in the first peak to elute) include 24∶1ω15, 24∶1ω13 and 24∶1ω11, which are not resolvable from each other on open-tubular GLC; 24∶1ω9 is followed by a peak for 24∶1ω7. The complex first 24∶1 isomer peak of fish oil fatty acids tends to coincide with or just follows the 22∶6ω3 peak in GLC analyses carried out on Carbowax-20M type open-tubular (capillary) columns.