During spermatogenesis, germ cells execute two meiotic divisions, then withdraw from the cell cycle and initiate postmeiotic differentiation. We show that the gene roughex (rux) is a dose-dependent regulator of meiosis II during Drosophila spermatogenesis. rux mutant germ cells execute the two meiotic divisions, but then undergo an additional M phase resembling an extra meiosis II. Conversely, germ cells with excess rux function fail to undergo meiosis II. rux does not appear to act directly at meiosis II. Rather, rux appears to act through cyclin A during premeiotic G2 to regulate meiosis II. We propose that cyclin A-cdc2 kinase at the G2 to M transition of meiosis I activates a target necessary for meiosis II, thereby coupling the two meiotic divisions.