1. The amino acid sequences of enzymes possessing enoyl-CoA hydratase activity have been aligned with each other and with the sequences of related proteins. 2. The amino acid sequences of enzymes possessing 3-hydroxyacyl-CoA dehydrogenase activity have been similarly aligned. 3. The N-terminal half of the 78 kDa gastrin-binding protein (Baldwin et al., J. biol. Chem. 261, 12,252-12,257, 1986) is related to the enoyl-CoA hydratase family, while the C-terminal half is related to the 3-hydroxyacyl-CoA dehydrogenase family. 4. Evolutionary trees for the two families are presented.