Reciprocal increases in rodent and ant densities on 0.1 ha plots from which the other taxon had been excluded demonstrate that these distantly related desert granivores compete for seeds. Relative to unmanipulated control plots, numbers of ant colonies increased 71% on plots where rodents were excluded; rodents increased 20% in numbers of individuals and 29% in biomass in the absence of ants. Comparisons of seed levels in the soil and of annual plant densities on experimental and control plots provide evidence that the rodent and ant populations are limited by and compete for food. Greater numbers of seeds and annuals occurred on plots where rodents and ants had been excluded than on plots where both taxa were present. Particular species of annuals were reduced in density by foraging of rodents. Ants increased species diversity by differentially harvesting seeds of the most common species. Results of these and other recent studies suggest that competition among distantly related organisms plays a major role in the organization of ecological communities.