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The flour beetles Tribolium confusum and Tribolium ccstaneum have been used in an extensive investigation of interspecies competition (Park, 1948). A finding of this study was that when no special precautions were takento eliminate the sporozoan parasite Adelina tribolii, T. confusum, as a single species, typically maintained populations about twenty to thirty per cent larger than did T. castaneim. No known differences in temperature, humidity, light, medium, or handling existed between these two series. In mixed-species populations, that is when there was competition between the two forms, T. castaneum was usually driven out by T. confusum. In the absence of the parasite, T. castaneum usually drove out T. confusum. There is some indication that in their natural distribution the two species vary in relative abundance according to latitude (Good, 1936) and to the breeding medium, whether wheat or flour (Miller, 1944). It is hoped that studies of competition may shed some light on such problems in the population ecology of the two species as well as on the general nature of interspecies competition itself. In many ways these beetles are excellent objects for the laboratory study of such phenomena. They inhabit the same ecological habitat from which they derive their entire food supply and in which their life-history, and that of successive generations. is passed. The DoDulations. includ