The rules ants follow when selecting seeds are important both to theories of colony organization and to the shaping of their impact as harvesters. Two Costa Rican forests yielded the most diverse harvester ant assemblages yet studied (41 and 444 species). I assayed seed size preferences within and between species using milled barley. Seed size selection varied at a number of levels. First, species differed in seed size use: small species mainly carried off small seeds; large species retrieved a broad range of seed sizes. Within species, one-third of those tested yielded inter-colony differences in preferred seed sizes. Finally, workers of polymorphic species tested showed significant worker/seed size correlations. Species, colony, and worker level differences are common among harvester ants. Their significance to community organization and colony ergonomics however is hotly debated and requires a better understanding of the dynamics of food limitation. From the plant's perspective, small seeds will be harvested by a wider variety of ants than large seeds.