The foraging ecology of two ant species inhabiting the same area in the Chihuahuan desert was investigated by study of conditions and magnitude of forager activity, estimation of forager population numbers, determination of critical maximum and minimum temperatures, and observation of foraging behavior. The foraging activity of Formica perpilosa was relatively independent of soil temperature and saturation deficits probably because of their ability to lose water without detriment and the fact that they forage in plants. Foraging activity in Trachymyrmex smithi neomexicanus ceased at saturation deficits greater than 35 gms/m3 and soil surface temperatures greater than 50 C. The mean critical thermal maxima and minima in F. perpilosa were 45.2 C and 5.2 C and in T. s. neomexicanus were 36.7 C and 9.7 C. The estimated forager population size in F. perpilosa ranged from 2,000 to 3,500 per colony. We estimated that the average forager population size in T. s. neomexicanus was bout one half that of F. perpilosa.F. perpilosa forage on both honey dew and insects. T. s. neomexicanus forage on leaf litter, and appear to exhibit a seasonal change in forage preference.