Functional and evolutionary dynamics of polymorphic esterases in catostomid fishes are discussed using Catostomus clarki from the Colorado River to develop a consideration of patterns of variation. Since esterase phenotypes among other closely related species provide support for some of the hypotheses formulated from investigations of C. clarki, brief mention is made of C. santaanae, C. platyrhynchus, C. discobolus, C. columbianus, C. plebeius, and C. insignis.