The foraging activity of 4 species of harvester ants as affected by soil surface temperature, saturation deficit at the soil surface, light and forage availability was studied in a desert grassland area 40 km NNE of Las Cruces, N.M. Pogonomyrmex sp. were capable of locomotor activity at temperatures between ca. 5 and 53 C and Novomessor cockerelli between 4 and 52 C. Pogonomyrmex sp. exhibited maximum foraging intensity at 45 C and N. cockerelli at 20 C. Forager population sizes varied among species: 1000-6000 in P. rugosus, ca. 1000 in P. californicus and N. cockerelli; and ca. 200-600 in P. desertorum. P. rugosus exhibited peak foraging activity and had the greatest number of colonies active following drought periods, ceasing foraging when granaries were filled, and foraged at night during midsummer. P. desertorum and P. californicus were strictly diurnal and did not exhibit hoarding behavior. P. desertorum responded to soil wetting by shifting activity between watered and unwatered controls directly in response to differences in soil surface temperature. Soil wetting stimulated intense activity in P. rugosus at a time when controls were inactive. Foraging in harvester ants seemed to be primarily affected by forage availability, secondarily by microclimate, with historical factors and interspecific competition as contributing parameters in some species.