The spatial and temporal distributions of 12 species of waterboatmen in a 1.2 ha pond in Northern Michigan were studied for three consecutive summers. Enclosure experiments involving the growth of one, two, and three corixid species were performed to test for interspecies competitive interactions and to determine whether the dominant species reach the upper limits to population growth. The experimental data indicate that interspecific competition between the two dominant species, Hesperocorixa lobata and Sigara macropala, in enclosures led to a signficant reduction in the population size of H. lobata but not of S. macropala. From the enclosure experiments it appears that lobata failed to grow to its maximum limit because of competition, while macropala did reach its limit, at least in late summer. The experiments also show that the early summer and late summer breeding habits of Hesperocorixa lobata and Sigara macropala, respectively, are not physiologically (genetically) fixed properties of these species. It is suggested that the corixid species ensemble here studied may be characterized as two sets of ecological homologues: one set of Hesperocorixa species that breeds early in the summer and one set of Sigara species that breeds later.