Tribolium castaneum and T. freemani produce sterile hybrid progeny in reciprocal crosses. The reciprocal crosses differ significantly in the mean numbers of progeny, progeny sex ratios, hybrid male body size and male antennal and leg morphologies. These results suggest an effect of either the X chromosome or the cytoplasm on characteristics of F1 hybrids. In contrast, large X chromosome effects on morphological traits are not usually observed in interspecific crosses among drosophilid flies. We also report large, significant differences in progeny numbers, body mass and degree of female bias in sex ratio between different geographic strains of T. castaneum when mated in reciprocal crosses with T. freemani. Sex ratio bias also varies significantly among matings within geographic strains of T. castaneum. When T. castaneum males are mated with T. freemani females, but not in the reciprocal cross, the F1 sex ratio is female biased, uncorrelated with family size and ranges from 57.14 per cent to 72.23 per cent female, depending on the geographic strain of the T. castaneum male.