A cDNA clone encoding a putative glyceraldehyde 3-phosphate dehydrogenase (GAPD-S) protein specific to spermatogenic cells was isolated from a mouse spermatogenic cell expression library. The Gapd-s cDNA contained 1451 bp of transcribed sequence, including an ATG initiation codon and a poly(A) addition signal. The location of the Gapd-s initiation codon differed from that of other Gapd sequences, resulting in a germ cell GAPD-S protein predicted to contain 105 additional residues at the amino terminus. While GAPD is constitutively expressed in somatic tissues, Northern blot analysis demonstrated that a Gapd-s probe hybridized to a 1.5-kb transcript present only in the testis. The Gapd-s mRNA was first detected during postnatal development in the testes of 20-day-old mice, suggesting that gene expression begins shortly after the appearance of haploid round spermatids. Northern analysis of RNA from isolated mouse pachytene spermatocytes and spermatids confirmed that Gapd-s expression is confined to post-meiotic germ cells. GAPD has been previously proposed to be the key enzyme regulating glycolysis in isolated round spermatids. We hypothesize that the GAPD-S enzyme plays an important role in regulating the switch between different pathways for energy production during spermiogenesis and in the spermatozoon.