The fatty acid composition of the seed lipids of plants, in contrast to leaf lipids, may contain highly specific unusual fatty acids, which are often correlated to plant family. For example, petroselinic acid is typical for the Apiaceae family, cyclopropene fatty acids are typical for the Malvaceae family, and cyclopentene-ring containing fatty acids for the Flacourtiaceae family. Other fatty acids may be characteristic for certain sub-families or only for certain species of plants within a family or genus. Gamma-Linolenic acid, DELTA5-18:3 and DELTA5-20:3 fatty acids and others may occur in several plant families, but are still linked to certain related species only. They can be analyzed by high-resolution capillary GLC. Particularly interesting is the situation in the family Ranunculaceae. GLC fingerprints of fatty acids are shown that may indicate a closer or less close relationship between species within this family.