In an enclosed lagoon at the Santa Barbara Campus of the University of California, medusae of the limnomedusiod hydrozoan Gonionemus vertens have been observed for 2 years, where they are found in the shelter of the green alga Enteromorpha, whose existence as thick surface mats in the winter and spring apparently determines their cyclic occurrence. Originally it was most likely introduced into the lagoon as an asexual polyp generation, transported from the north either as part of the fouling community on boat hulls or wafted on flotsam or detached seaweed by the California Current. Thus, as tiny polyps it may have remained undetected for years before conditions became favorable for the establishment of a sexual generation of medusae. Also, the introduction of polyps or medusae into the lagoon by humans is quite possible, although neither onto-genetic stage has been used as a laboratory animal in the U.C.S.B. Marine Laboratory. The lagoon protects it from the ravages of predation and tidal flushing, which may have precluded its colonization of local estuaries.