We examined the spatial pattern of the ant Myrmecocystus mexicanus Wesmael. Although intraspecific dispersion is highly uniform, colonies were significantly associated with reproductively mature nests of the harvester ant Pogonomyrmex occidentalis (Cresson). Colonies of M. mexicanus were more likely to be found within 3 m of P. occidentalis and less likely to be found as far as 10 m away. The protein component of the diet of M. mexicanus at this site is almost exclusively dead or moribund workers of P. occidentalis. M. mexicanus appears to associate with one of its consistent food sources.