Studies of the variation in productivity in inbred strains of Tribolium confusum and Tribolium castaneum were undertaken to determine the effects of inbreeding on different strains of common ancestry. Six strains of each species were tested after eight to ten generations of inbreeding by full sibling crosses. The founding parents of each strain were taken from synthetic populations, on of T. confusum, another of T. castaneum, each of which had been established by systematic crosses involving stocks of different origin. Three pairs of sibling strains, two of T. confusum, one of T. castaneum, each pair derived from the same ancestral parents in the founding generation of inbreeding, were included in the 12 stains investigated. In length of productivity period, significant interstrain variation in means and variances was found for the strains of T. castaneum investigated. Differences between the strains of T. confusum examined were found to be not significant. In total 120—day productivity, significant intraspecies, between—strain variation in means and variances was found for both T. confusum and T. castaneum. Analysis of per—day productivity for a 40—day period revealed significant strain differences in productivity in T. confusum; interstrain differences in variance could not be demonstrated. In T. castaneum, significant interstain variation in means and variances was found. Interstrain differences in productivity reflect genotypic differences due to inbreeding as well as genotypic differences of the founding parents of the different strains. Differences between sibling strains, having the same founding parents, are attributable to inbreeding only. The two sibling strains of one pair T. confusum tested exhibited remarkable similarity in all traits compared. The other pair of T. confusum showed slight though significant differences only in total 120—day productivity period and per—day productivity. Difference in total productivity fell short of significance, but the difference in variances was significant. Apparent interstrain variation is greater in T. castaneum than in T. confusum for all three traits analyzed. T. castaneum appears to perform relatively better and T. confusum relatively less well in competition experiments than would be indicated by their basic productivity.