Caenorhabditis elegans spermatid formation involves asymmetric partitioning of cytoplasm during the second meiotic division. This process is mediated by specialized ER/Golgi-derived fibrous body-membranous organelles (FB-MOs), which have a fibrous body (FB) composed of bundled major sperm protein filaments and a vesicular membranous organelle (MO). spe-39 mutant spermatocytes complete meiosis but do not usually form spermatids. Ultrastructural examination of spe-39 spermatocytes reveals that MOs are absent, while FBs are disorganized and not surrounded by the membrane envelope usually observed in wild type. Instead, spe-39 spermatocytes contain many small vesicles with internal membranes, suggesting they are related to MOs. The spe-39 gene was identified and it encodes a novel hydrophilic protein. Immunofluorescence with a specific SPE-39 antiserum reveals that it is distributed through much of the cytoplasm and not specifically associated with FB-MOs in spermatocytes and spermatids. The spe-39 gene has orthologs in Drosophila melanogaster and humans but no homolog was identified in the yeast genome. This suggests that the specialized membrane biogenesis steps that occur during C. elegans spermatogenesis are part of a conserved process that requires SPE-39 homologs in other metazoan cell types.