The relationship between interspecific competition and a variety of community phenomena is emphasized as well as the relationship of competition to species divergence and association. Sources of thought and information concerning interspecific competition are identified and the contributions of each are briefly summarized. The need for field studies of competition is pointed out and some of the problems associated with such studies are discussed. It is proposed that the Gyrinidae are highly suited for studies of competition, especially the species of eastern North America. Three species of the genus Dineutes were selected for the present study. Field and laboratory methods employed in the present study are described. An outline of the natural history of the species of the genus Dineutes is presented. All three species are similar in this respect. Dineutes assimilis has a shorter breeding season than either of the other two species. Dineutes assimilis and D. nigrior commence breeding slightly earlier than D. horni. The similarity of the three species is indicated by making several comparisons. These comparisons involved labial width ratios, fecundity, and the rate of larval development and growth. The labial width ratios suggest that the species should not coexist on the basis of Hutchinson’s (1959) concept of size separation. Field distribution patterns of the three species are considered at several levels including: distribution in North America, distribution in Michigan lakes, distribution in Michigan ponds, and dispersion within the same lake. A general pattern of habitat segregation is revealed but instances of coexistence are also present. The behavioral interactions of larval gyrinids are described throughout the period of larval growth. Food-dependent cannibalism is a major feature of larval behavior. The results of experiments involving intraspecific and interspecific competition are presented. Larval cannibalism functions as a mechanism for adjusting population size to the existing food supply. The species are different with respect to this cannibalistic adjustment of population size and these species differences explain the outcomes observed in some of the interspecific experiments. The outcome of competition is influenced by temperature, density or initial relative abundance. The effectiveness of larval cannibalism as a mechanism for adjusting population size is discussed. It is though that this mode of regulation is suited to the efficient utilization of a fluctuating food supply. The hypothesis that competition is an explanation for the observed field distribution patterns is put forth. Cases of obvious coexistence are thought to result from the indeterminancy in outcome of competition caused by intermediate environments, possibly in conjunction with heterogeneous environments. The results of competitive interaction among these species in nature are considered and it is thought that the species populations may have undergone evolutionary changes because of competition. Habitat selectoin by the imagoes pay be one result of such change. Geographical differences in habitat occupancy are considered and it is proposed that the species D. horni and D. nigrior display a form of geographical variation with respect to their tolerance for competition with one another.