Harvester ants play an important ecological role as seed consumers in arid areas. We performed choice experiments to study preferences of Pogonomyrmex rastratus (Mayr), Pogonomyrmex mendozanus (Cuezzo & Claver), and Pogonomyrmex inermis (Forel) (Hymenoptera: Formicidae) for seeds of six native species (three grasses, two forbs, and a shrub) in the central Monte desert, Argentina. We hypothesized that ant diet composition would reflect ant preferences. Thus, based on previous ant diet studies, we expected that 1) the three species would prefer grass to nongrass seeds, 2) P. inermis would have a lower preference for nongrass seeds than P. rastratus and P. mendozanus and 3) relative abundance of seeds in the diet would be positively associated with relative preference. In general, relative abundance of seeds in the diet was positively associated with relative preferences. Ants preferred grass seeds, but P. inermis did not have a lower preference for nongrass seeds. We also analyzed the relationship between preferences for seeds of the six species and their size and morphology, and we found higher preferences for seeds of intermediate size but no relationship with seed morphology. The overall match between seed preferences and diet composition could increase the chances of ants affecting the abundance and composition of some seed resources in the Monte desert, with important community implications.