It is well known that 24-hour day-night (light-dark) cycles can entrain the circadian rhythms of most species possessing circadian clocks. However, much less is understood about how other environmental cycles operate as entraining signals (zeitgebers) to synchronize (entrain) the internal clock with the outside world. Potential non-photic zeitgebers include daily cycles of temperature, food availability, and social signals. This project’s goal is to evaluate the efficacy of temperature cycles of varying amplitudes, ranging from 1°C to 10°C, as potential zeitgebers for two different circadian rhythms, eclosion and locomotor activity, in the flesh fly (Sarcophaga crassipalpis). Both rhythms were monitored in individual flies, using infrared motion detectors, under precisely controlled 24-hour temperature cycles (12 hours of high temperature [thermophase] alternating with 12 hours of low temperature [cryophase]) in constant darkness. Our results show clear entrainment of eclosion, a once-in-a-lifetime event, and locomotor activity, reflecting daily sleep-wake rhythms, in response to temperature cycles at amplitudes of 2.5, 5, and 10o C. At 1o C amplitudes, the evidence indicates a partial effect of temperature on the phasing of the two behaviors but not true entrainment (phase regulation), suggesting that 1o C amplitudes may be near the threshold for detection by the entrainment pathways communicating with the circadian clock. Interestingly, although both light and temperature cycles entrain the locomotor activity rhythm, the entrainment profiles are remarkably different, thereby suggesting that light and temperature cycles activate different behavioral programs. Finally, flies placed under different constant temperatures have endogenous circadian periods of locomotor activity that are remarkably similar, indicating a high level of temperature compensation. Our results provide conclusive evidence that temperature is a strong circadian zeitgeber in flesh flies, thereby expanding the known repertoire of environmental cues these organisms use to sync their internal clock with the world around them. These findings also set the stage for future experiments designed to explore the interactions between light and temperature entrainment mechanisms – these zeitgeber interactions almost certainly occur in nature but have received little or no attention.