The average diameter of the seminal vesicles is reduced to approximately 2/3 of wild-type in homozygous males. Later spermatid nuclei are out of register, in contrast to wild-type where the sperm heads are in close lateral alignment. No motile sperm are seen. Sperm are not transferred to the female reproductive system during copulation, although copulation does occur. Nuclear elongation is variable and correlates with the degree of development of nuclear microtubules in developing spermatids. Most stage 7 nuclei have condensed chromatin concentrated along the inner side of the nuclear envelope in a thick and apparently randomly oriented layer. These nuclei do not elongate and no closely associated microtubules are seen over the outer nuclear surface. The few more nearly normal nuclei do have a layer of microtubules associated with the outer nuclear surface, but the tubules are less numerous than normal. In the vicinity of these microtubules, the chromatin condensation at the inner nuclear surface is similar to that seen in wild-type spermatids. The condensed material is characterised by a series of ridges on the outer surface which correspond in period to the spacings between the microtubules in the surrounding sheath. Cross-sections of sperm bundles show up to 91 axonemal cross sections (in contrast to the wild-type 60 to 64) and as many as 185 paracrystalline structures per bundle. Bundles showing greater than normal numbers of tail structures contain axonemes with reversed doublet orientations. This suggests that some of the axonemes are folded. Mutant bundles often contain numerous dissociated axonemal components linked in normal fashion with the smooth endoplasmic reticulum (ER). The structure of isolated components which are not associated with the smooth ER is frequently aberrant. Occasionally, two sets of central doublets are seen in axonemes that are at least partially disrupted. Aberrant bundles often contain Nebenkern derivatives in which as many as four paracrystalline bodies are seen (usually one or two are seen). A paracrystalline structure is seen at almost all points where the Nebenkern derivative contacts the smooth ER. The acrosome appears to differentiate normally, but is sometimes contorted, due, perhaps, to its association with abnormal nuclear configurations. The spermatids differentiate to stage 10. Degeneration is subsequently seen. No intact sperm are seen in the seminal vesicles.