Perognathus penicillatus and Perognathus intermedius were studied in two areas in Dona Ana County, New Mexico, from 1970-1972. Perognathus penicillatus was associated with sandy soils and P. intermedius with soils composed of predominantly larger particles. Soils from areas where P. intermedius was found had a lower heat buffering capacity than soils from areas occupied by P. penicillatus. Artificial burrows when occupied by either species had a higher concentration of CO2 in P. intermedius type soils. Perognathus penicillatus occupied burrows in cooler places, survived a narrower temperature range, and exhibited higher rates of evaporative H2O loss with increased ambient temperature and CO2 concentration than did P. intermedius. Perognathus penicillatus was the more aggressive species, and dominated P. intermedius in laboratory encounters.