Homozygous males have an apparently normal reproductive system. Large numbers of primary spermatocytes and other early developmental stages are seen in the testes of males as old as 10 days, few elongate bundles approaching the wild-type length are seen and elongate sperm heads are not present. No sperm are transferred to the female reproductive tract during copulation. Both normal and disrupted axonemes are seen in sections of mutant spermatid tails. Axonemes that are normal at one level are probably disrupted at more distal levels. No disrupted centrioles have been observed. Spermatids within developing bundles lack the normal close registering seen in wild-type bundles. Spermatids which have more nearly normal axonemal complexes also have more nearly normal mitochondrial derivatives. In spermatids in which the axoneme is disrupted, the mitochondrial derivatives are generally more widely dispersed in the cytoplasm and more irregular in shape than normal. The derivatives generally fail to narrow and do not elongate normally. The scattering of doublets is random in the disrupted axonemes, but the doublets are always found in association with the cytoplasmic membranes. Paracrystalline bodies form within the primary mitochondrial derivatives at each point of contact between the cytoplasmic membranes and the membranes of the derivative. The large and irregular primary derivatives therefore often contain several paracrystalline bodies, each of which increases in size to approximately that found in normal mature spermatids. The elements of the axonemal complex differentiate even though the basic pattern is disrupted and the components are scattered throughout the cytoplasm. Nuclei undergo some condensation and elongation, but development stops prior to head maturation. The degree of abnormal development varies both within a single bundle and between bundles. Mutant spermatids generally have a greater cross-sectional area than normal spermatids.