We have identified a novel Drosophila gene, degenerative spermatocyte (des), that may be required for the initiation of meiosis in spermatogenesis. In des mutant testes, the primary spermatocytes become mature in size but degenerate without initiating meiotic chromosome condensation in their nuclei. We have mapped the locus of the des gene at 26A on the left arm of the second chromosome, and identified two related, differentially spliced transcripts (des-1, des-2) from the locus. The genomic DNA fragment encoding both transcripts is able to rescue the des phenotype when introduced into mutant flies. The expression of the des-1 transcript is significantly suppressed in the des mutant testes, whereas expression of the des-2 transcript is undetectable in both wild-type and mutant testes, indicating that the des-1 transcript encodes the des function in spermatogenesis. The des-1 transcript is selectively expressed in primary spermatocytes during normal spermatogenesis. The nucleotide sequence of the des-1 transcript predicts that it encodes a novel transmembrane protein. These results raise the possibility that the des gene product may be required for interactions between primary spermatocytes and surrounding somatic cells.