Cells of the fission yeast Schizosaccharomyces pombe carrying the mei3 mutation are unable to initiate meiosis, being arrested before premeiotic DNA synthesis. A plasmid pDB(mei3)1 containing an 8.6 kilobases (kb) DNA insert which complemented mei3 mutations was isolated from an S. pombe genomic library constructed in the vector pDB248′. A HindIII fragment of 2.6 kb subcloned in both orientations into pDB248′ complemented the mei3 mutation. The plasmid was designated pDB(mei3)2. The 2.6 kb fragment ligated to the vector YIp32 was integrated into the S. pombe chromosome at the mei3 locus, indicating that the cloned DNA fragment contains the mei3 gene itself. Both pDB(mei3)1 and pDB(mei3)2 allowed partial recovery of meiotic and sporulation ability in mei1 mutants, suggesting a close relationship between the mei1 and mei3 genes. Northern blot analysis with pDB(mei3)2 as the probe indicated that the mei3 mRNA (1.3 kb in length) is more abundant in cells cultured in nitrogen-free sporulation medium than in nitrogen-rich growth medium.