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This report reexamines experimentally the problem of competitive indeterminacy in mixed-species pop-ulations of the flour beetles, Tribolium confusum and T. castaneum. Indeterminacy takes the form of alternative competitve 'outcomes: in some replicate cultures one species exterminates the other with a probability, say p, whereas in others, the opposing species wins with a complementary probability, 1 -. The conventional explanation for this is the genetic foundef effect hypothesis-an explanation based on genetic stochasticity. The experiment reported here partitioned indeterminacy into founder effect and nonfounder effect components. The results implicate demographic stochasticity, not classical genetic founder effect, as a Factor influencing the identity of the winning species. This report presents findings from an experiment designed to resolve the biological mechanisms underlying competitive indeterminacy in mixed-species populations of the flour beetles, Tribolium castaneum and T. confusum. A detailed account will be published later.Both flour beetle species are pests of stored grain products, particularly flours, and have become model organisms for laboratory investigations of interspecies competition. Thomas Park (1) first reported indeterminacy in this competitive system in 1948, and it has been a recurring phenomenon ever since in studies in his own laboratory (2-6) and in the experiments of I. M. Lerner, P. S. Dawson, and their associates (7-11). There are several published reviews of this work (12)(13)(14)(15)