Fatty acid analyses of seed lipids in 46 species of Cuphea are presented, representing the first major survey of a molecular nature for the family A remarkable diversity in composition is found, with seeds containing high amounts of several medium chain fatty acids. Lauric acid (12:0) predominates in 43% of the species studied, constituting 50-74% of the total fatty acid content. Capric acid (10:0) is the dominant fatty acid in 32% of the species, comprising as much as 87% of the total acid content. Caprylic acid (8:0) predominates in one section of the genus. The emphasis on production of fatty acids with carbon chain lengths of 12, ten, and eight carbon atoms is unique among plant genera studied to date. Among seven of the nine sections studied, one pattern of fatty acid composition predominates. Two sections have no characteristic pattern, supporting other evidence of their polyphyletic origin. The most significant systematic contribution is made by comparison of the predominate fatty acid components in the seed lipids. When used in conjunction with floral morphology, pollen studies, and chromosome number, it provides an important new basis on which to draw inferences of evolution and clarify present relationships within the genus. Additionally, a trend from the longer-chained, unsaturated linoleic acid (18:2) as a major lipid component to shorter-chained saturated capric and caprylic acids is correlated with increasing floral specialization. It is suggested that mutations in regulatory genes have occurred which cause fatty acid production in seeds to cease at progressively earlier stages, resulting in accumulation of large amounts of single fatty acids of progressively shorter carbon chain lengths.