Electrophoretic variation at twenty gene loci, patterns of behavioral thermoregulation, and genotype-specific malate dehydrogenase kinetics were investigated among populations of the red shiner, Notropis lutrensis, and the blacktail shiner, N. venustus, collected from thermally altered and thermally unaltered portions of their ranges. Genic variation was found to be high among red shiners and low among blacktail shiners. The behavioral response of the blacktail shiner to temperature was fixed among the populations sampled, whereas the response of the red shiner was mutable. Finally, blacktail shiners have incorporated into their genome an Mdh-B allele which functions well at low temperatures; red shiners, displaying high levels of Mdh-B polymorphism, maintain a more complex set of allozymes which function well over a wide range of environmental temperatures. These data are consistent with reported ecotypic distributions of the species in Texas waters; i.e., blacktail shiners occur in cool, thermally static habitats, and red shiners are tolerant of wide temperature ranges.