Community structure in backswimmers (Hemiptera: Notonectide), was investigated via extensive sampling throughout southern Arizona, USA, and Sonora, Mexico, from 1980 through 1985. Co-occurrence and relative abundance data were collected in more than 65 ponds, and in 177 rock basin pools (tinajas) in 21 canyons in the Southwest. Eleven species were collected in Arizona and Sonora, and were divided into two groups, species found in ponds and species found in tinajas. Only two species occurred significantly in both habitats. Tinaja species are largely Southwst endemics, and pond species are widespread or tropical in distribution. Data from artificial habitats suggest that the tinaja species use relatively high topographic relief, and pond species use relatively large surface area as cues to find their respective habitats. Two body size patterns are consistent with a competition explanation of local community structure. The body sizes of co-occurring species are relatively evenly distributed among species occurring in pond and tinaja habitats, and species of similar body size tend not to co-occur (body size ratio <1.3). For example, Notonecta kirbyi and N. lobata only co-occur in tinajas at intermediate elevations; lobata is absent at high elevations and kirbyi is absent at lower elevations. N. indica occurs in ponds at lower elevations and N. unifasciata occurs at higher elevations. Buenoa hungerfordi and B. arizonis both occur in tinajas, but not at the same time of year. Predation was shown experimentally not to be important in producing the body size pattern. Notonecta spp. preyed heavily on the smaller of two Buenoa species presented, an effect that would act to reduce the community-wide body size ratio. Because notonectid communities have larger body size ratios than expected by chance, predation would seem not to be involved in producing this pattern. However, predation does appear to reinforce microhabitat partitioning between the two genera in that Buenoa occupy deeper portions of the water column in the presence of Notonecta than in their absence. This further displaces coexisting individuals of the two genera in space, and reduces overlap in foraging for aquatic insect prey and promotes coexistence.