A cDNA encoding the Arabidopsis extraplastidic linoleate desaturase (FAD3) was overexpressed in the seeds of wild-type Arabidopsis and in a mutant line that accumulates high levels of oleic acid. In the transformed wild-type plants, linolenic acid (18:3Delta9,12,15) increased from 19% to nearly 40% of total seed fatty acids, with a corresponding decrease in linoleate content (18:2Delta9,12). In the high oleate mutant, a large increase in the level of a fatty acid identified by gas-chromatography/mass-spectrometry as mangiferic acid (18:2Delta9,15) was observed. The results demonstrate that the polymethylene-interrupted dienoic fatty acid, mangiferic acid, can be produced in seed oil through the overexpression of a fatty acid n-3 desaturase.