Normal distribution of chromosomes to daughter cells is insured by the proper functioning of the spindle. Homozygosity for a semi-lethal mutation of Drosophila melanogaster (abnormal spindle) altering this structure has the following effects: the mitotic cycle is arrested in metaphase, leading to a high frequency of polyploid cells; sex chromosome disjunction during male meiosis is severely affected, as revealed by the resulting exceptional (diplo and nullo) gametes (microscopic examination of spermiogenesis confirms this aberrant segregation); meiotic spindles of living cells are morphologically abnormal; and tubulins extracted from mutant larvae are normal in amount, electrophoretic mobility, and ability to form microtubules in vitro. The results suggest that the mutant phenotype is due to an altered structural component of the spindle other than tubulins.