Dipodomys merriami collinus and Dipodomys agilis cabezonae are two heteromyid rodents that occur in partial sympatry on the western edge of the Colorado Desert in California. Activity patterns, interspecific behavior and water requirements were investigated in the laboratory and correlated with the relative distributions of these species in the field. D. m. collinus and D. a. cabezonae were trapped in a vegetation cline along an altitudinal topographic gradient. There was a reversal of relative abundance of these two species along this cline. This study revealed apparent subspecific differences in the ability of D. agilis to produce concentrated urine, but no apparent differences under the midly stressful environmental conditions were noted between D. agilis and D. merriami .