The effect that harsh, variable desert temperatures might have on the structure and population dynamics of a guild of Bruchidae (Coleoptera) that feed in the seeds of mesquite,Prosopis velutinaWooten, was examined in a 3-yr field and laboratory study. Metabolic rate, fecundity, longevity, developmental times, survivorship, and body size were measured across the temperature spectrum in which the species normally live. The 4 species that compose the guild—Algarobius prosopis(LeConte),Mimosestes amicus(Hom),Mimosestes protractus(Hom), andNeltumius arizonensis(Schaeffer)—divide the use of the resource temporally. The first 2 species dominate resource use and overlap entirely in time, whereas the 2 latter minor species utilize opposite ends of the temporal resource spectrum. Of the two dominants,M. amicusfunctions as a physiological generalist, apparently sacrificing resource adaptation for greater temperature adaptation and very high reproductive output. In contrast,A. prosopisiswell adapted to both the use of mesquite as a resource and also to the desert thermal environment. The 2 minor species seem to be less well adapted to both the resource and the environment. Temperature clearly plays a strong role in determining the structure of this guild of bruchids.