twine is the second homolog of the fission yeast gene cdc25 to be found in Drosophila. Both string and twine cDNAs can rescue a temperature-sensitive cdc25 mutation in fission yeast, but not a deletion. We detect the expression of string but not twine transcripts in the proliferating cells of newly cellularized embryos, in third instar larval brains, and in imaginal discs. Both genes are abundantly expressed in nurse cells during oogenesis, the maternal transcripts persisting throughout the syncytial stage of embryonic development. In the testis, twine transcripts are seen in the growing stage of premeiotic cysts. Analysis of a twine mutant suggests a requirement for the gene during oogenesis, during syncytial embryonic development, and for male meiosis. Meiosis does not occur in homozygous twine males, which produce cysts containing 16 rather than 64 spermatids.