Results are reported from a genetic study of hybrid inviability and three 'fertilization traits' (sperm motility and length, and testis size) that affect hybrid sterility between the sibling species Drosophila simulans and D. sechellia. The main findings are as follows. (i) For sperm length there was a dominant effect of the D. simulans genome over that of D. sechellia, and the Y chromosome of D. sechellia in the background of D. simulans reduced the sperm length. (ii) In contrast, testis length, in spite of its generally high correlation with sperm length, showed an additive effect. (iii) We found a strong asymmetric incompatibility between the D. sechellia X chromosome and D. simulans autosomes: D. sechellia X chromosome with D. simulans autosomes, but not the reverse, showed a significant reduction in testis length as well as in hybrid inviability compared to the parental species. (iv) Between the two autosomes, chromosome 3 had a greater effect on these traits than chromosome 2, and there was additionally an epistatic effect between these chromosomes with respect to their parental vs. recombinant status: recombinant chromosomes 2 and 3, together, had lower viability than any other combination. (v) The testis size in the backcross generation was greater than the parental species, suggesting that some modifier genes are being released from their species-specific genetic control. (vi) The species-specific homogeneity of the genome was important for all three traits--offspring viability, hybrid male fertility and testis length. These results are discussed with respect to the role of sexual selection and genetic divergence during speciation.