The role of the zooxanthellae in the life cycle of Cassiopeia andromeda was reexamined. Symbiotic and aposymbiotic polyps were maintained at various temperatures between 17° and 30°C, and numbers of buds and ephyrae formed at each temperature were recorded.The number of buds formed by the symbiotic polyps was inversely related to temperature. After 20 days at above 20°C, only ephyrae were formed.In aposymbiotic polyps, buds were formed at all temperatures tested. Strobilation occurred above 25°C, and aposymbiotic ephyrae were obtained. Aposymbiotic polyps reinfected with zooxanthellae formed ephyrae at 22°C. A similar effect was obtained by repeatedly injecting aposymbiotic polyps with heat- or cold-inactivated homogenate derived from adult medusae. No reinfection occurred in the latter.It is concluded: 1. At higher metabolic rates, strobilation supersedes budding in C. andromeda 2. A higher metabolic rate is obtained at elevated temperatures, and by a biotrophic effect of the symbiotic zooxanthellae. 3. ...