The desert-grassland whiptail (Cnemidophorus uniparens) is an  all-female lizard species that reproduces clonally by parthenogenesis. Here we report marked geographic variation in the aggressive behavior of paired individuals in which pairs consisted either of individuals from within study sites or individuals representing each of two different study sites. Although we document allozyme variation within each study site, this did not differ significantly between sites. Previously reported restriction endonuclease analysis of mitochondrial DNA indicates that the two lizard populations used in the present study arose from the same or closely related maternal ancestors by interspecific hybridization. Gross climatological differences do not appear to explain the behavioral variation between sites. The possible roles of polygenic effects not detected by the biochemical analyses and laboratory ies of environmental effects on the development of aggressive behavior await further investigation.