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.Pogonomyrmexsp. were capable of locomotor activity at temperatures between ca. 5° and 53°C andNovomessor cockerellibetween 4° and 52°C.Pogonomyrmexsp. exhibited maximum foraging intensity at 45°C andN. cockerelliat 20°C. Forager population sizes varied among species: 1000–6000 inP. rugosus, ca. 1000 inP. californicusandN. cockerelli; and ca. 200–600 inP. desertorum. P. rugosusexhibited 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. desertorumandP. californicuswere strictly diurnal and did not exhibit hoarding behavior.P. desertorumresponded 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 inP. rugosusat 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.