The ability of a river to alter gene flow in the lizard Sceloporus undulatus was investigated. The Cimarron River of northern Oklahoma was found not to be a barrier to gene flow but instead may actually facilitate movement within this species. Populations from alternate sides of the river were not genetically different whereas populations from the river's edge were found to be more genetically homogenous than inland populations, indicating that gene flow is enhanced in riverine areas. This gene flow along the river tends to be asymmetric in the direction of the river's current, possibly aided by rafting of lizards or their eggs downstream during floods. Two factors which appear to promote differentiation in this species are the magnitude of geographic distance between populations and differences in habitat. An east-west genetic cline was demonstrated, and coincides well with a similar cline in habitat. In addition, far eastern Oklahoma samples consisting of pure S. u. hyacinthinus were found to be strongly differentiated from all other samples, indicating that this subspecies is genetically quite different from S. u. garrnani.