A new fatty acid isolated from the scallop Pecten maximus was determined to be a 22:4 geometrical isomer [Δ4?,7?,10?,13?] (X) using GC-MS analysis of the 2-alkenyl-4,4-dimethyloxazoline derivative. The number of trans double bonds was determined on silver ion HPLC by comparison with the mono-trans geometrical isomers of 22:4[Δ4cis,7cis,10cis,13cis] (S), obtained through partial hydrazine reduction of all-cis-22:6(n-3) followed by a p-toluenesulfinic isomerization of the derivatives. Comparing the trienes obtained through partial hydrazine reduction of (X) and (S) determined a cis-4,7,10,trans-13-docosatetraenoic fatty acid. This fatty acid has not been reported previously in any organisms and is believed to be specific to the pectinid family. Copyright (C) 1999 Elsevier Science B.V.