ABSTRACT The aly, can, mia and sa genes of Drosophila are essential in males both for the G2-meiosis I transition and for onset of spermatid differentiation. Function of all four genes is required for transcription in primary spermatocytes of a suite of spermatid differentiation genes. aly is also required for transcription of the cell cycle control genes cyclin B and twine in primary spermatocytes. In contrast can, mia and sa are required for accumulation of twine protein but not twine transcript. We propose that the can, mia and sa gene products act together or in a pathway to turn on transcription of spermatid differentiation genes, and that aly acts upstream of can, mia and sa to regulate spermatid differentiation. We also propose that control of translation or protein stability regulates entry into the first meiotic division. We suggest that a gene or genes transcribed under the control of can, mia and sa allow(s) accumulation of twine protein, thus coordinating meiotic division with onset of spermatid differentiation.