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1. Temporal 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.2. Seeds were the only numerically important type of food in the diets of all species.3. The ants partitioned the resource according to both seed species and seed size, although there was considerable overlap. 4. Pheidole xerophila had 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.5. When the abundance of the small seeds of BouteZoua barbata decreased, the middle-sized ant, Veromessor pergarzdei, showed a decrease in foraging activity, increase in niche breadth, and a decrease in overlap with P.xerophiIa.6. Seed size preferences of Kpergandei did not vary seasonally, except that during the month of highest seed abundance, Kpergandei showed no size preference.7. Pogonomyrmex rugosus was 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. 8. We suggest that nocturnal foraging by P.rugosus during the summer months was a response to interference with diurnal foraging by either predation frorn horned lizards or competition from V.pergandei.9. Seasonal abundance of small seeds explains most of the seasonal foraging patterns of P.xerophiZa and V.pergandei. The summertime abundance of larger seeds during years of adequate precipitation may account for the seasonal activity patterns of P.rugosus.