The majority of fsco^Z1376 mutant spermatids have one large nebenkern with four nuclei of normal size, indicating a cytokinetic failure in both meiotic divisions. A small proportion (3%) have two nuclei and a large nebenkern, indicating failure of only one of the meiotic divisions, while 2% of spermatids contain three nuclei, which may be due to failure of cytokinesis in the first meiotic division, followed by budding off of only one daughter cell in the second meiotic division. Further, some spermatids (4% of cases) have more than four nuclei, which may result from defects in the mitotic divisions preceding mitosis. fsco^Z1376/Df(3R)p25 mutant spermatids show the same phenotype but a greater number have more than four nuclei compared to fsco^Z1376 homozygotes. During early telophase of fsco^Z1376 spermatocytes, there is an actin ring of normal appearance and and a normal central spindle. However, as telophase progresses the actin ring fails to fully constrict and the central spindle either becomes less dense or completely disorganized. The cleavage furrow initiates ~10 minutes after anaphase onset as in wild type, but shows a limited ingression and then regresses leading to a failure of cytokinesis. fsco^Z1376/Df(3R)p25 mutants display the same phenotype.