The sea urchin Tripneustes ventricosus is a common, yet relatively poorly known, grazer of seagrass beds and coral reefs throughout the Caribbean. We compared the size and abundance of urchins between adjacent seagrass and coral reef habitats (where macroalgae are the dominant primary producers). We also conducted a laboratory experiment comparing the growth rate of juvenile urchins fed a diet of either macroalgae or seagrass. Reef urchins had signiWcantly larger test diameter than those in the seagrass on some sampling dates. This size diVerence may be at least partially explained by diet, because laboratory-reared urchins fed macroalgae grew signiWcantly faster than those fed seagrass. The seagrass population, however, was stable over time, whereas the reef population exhibited strong Xuctuations in abundance. Overall, our study indicates that both the seagrass and coral reef habitats are capable of supporting healthy, reproductive populations of T. ventricosus. Each, however, appears to oVer a distinct advantage: faster growth on the reef and greater population stability in the seagrass.