Cyclin proteins are thought to trigger entry into mitosis. During mitosis they are rapidly degraded. Therefore, mitosis and consequently cyclin degradation might be triggered at a time when cyclins have reaccumulated to a critical level. We cloned and sequenced a Drosophila cyclin A homolog and identified mutations in the corresponding gene. Immunofluorescent staining revealed that cyclin A accumulates in the interphase cytoplasm of cellularized embryos, but relocates to the nuclear region early in prophase and is completely degraded within metaphase. Cyclin A was expressed in dividing cells throughout development, and a functional cyclin A gene was required for continued division after exhaustion of maternally contributed cyclin A. Importantly, the timing of post cellularization divisions was not governed by the rate of accumulation or level of cyclin A.