X/Ab(Y)Y146/Dp(1;Y)y+ males are sterile. Testes show a departure from normal development at early postmeiotic stages of spermatid differentiation. Before nuclear elongation, round spermatid nuclei begin to fall apart in a large number of spermatid bundles, resulting in singular nuclei heads that are dispersed throughout the tails. In approximately 10% of males, the testes contain exclusively round spermatids scattered throughout the length of the tails. Many of the spermatid bundles with scattered nuclei are much smaller in diameter than normal, suggesting that some of the spermatid tails fail to develop. However, some spermatid bundles show aligned nuclear heads associated with individualisation complexes, as in wild type. C(1;YS)1/Ab(Y)Y146 males are fertile. Ab(Y)Y146 is not complemented by Ts(1Rt;YSt)V8. Ab(Y)Y146 is complemented by Ts(1Rt;YSt)W19. Ts(1Lt;YSt)P7/Ab(Y)Y146 males produce large numbers of progeny, comparable to wild-type X/Y males. Ts(1Lt;YSt)W27/Ab(Y)Y146 males produce large numbers of progeny, comparable to wild-type X/Y males. Ts(1Lt;YSt)V24/Ab(Y)Y146 males show a significant reduction in male fertility compared to wild type; 19% of males fail to produce any progeny, 46% produce very small numbers of progeny (less than 40/male) and only 10% produce more than 80 progeny/male. Postmeiotic defects are seen in the testes; spermatid bundles with scattered singular nuclear heads are seen frequently, but account only for a small proportion of spermatid bundles. Some individualisation complexes are located away from the nuclear bundles, apparently resulting from caudal movement along the tails (as occurs in wild type). Individualised spermatids are seen in the basal region of the testes and mature sperm are seen in the seminal vesicles.