ABSTRACT.1Temporal patterns of seed use were studied from late winter to autumn in three species of seed‐harvesting ants in the Sonoran Desert. Measures of effective foraging activity, dietary niche breadth and dietary niche overlaps were obtained each month and were tested for correlation with estimates of the available seed resource.2Seeds were the only numerically important type of food in the diets of all species.3The ants partitioned the resource according to both seed species and seed size, although there was considerable overlap.4Pheidole xerophilahad the smallest forager body size and is a specialist on small seeds because it harvested them in greater proportion than their rank in the soils and expanded its diet to larger seeds only when the abundance of small seeds declined.5When the abundance of the small seeds ofBouteloua barbatadecreased, the middle‐sized ant,Veromessor pergandei, showed a decrease in foraging activity, increase in niche breadth, and a decrease in overlap withP.xerophila.6Seed size preferences ofV.pergandeidid not vary seasonally, except that during the month of highest seed abundance,V.pergandeishowed no size preference.7Pogonomyrmex rugosuswas the largest ant; it preferred larger seeds and was inactive when small seeds were most abundant. Seasonal foraging activity and niche parameters were random in relation to seed abundance.8We suggest that nocturnal foraging byP.rugosusduring the summer months was a response to interference with diurnal foraging by either predation frorn horned lizards or competition fromV.pergandei.9Seasonal abundance of small seeds explains most of the seasonal foraging patterns ofP.xerophilaandV.pergandei.The summertime abundance of larger seeds during years of adequate precipitation may account for the seasonal activity patterns ofP.rugo