1. A method was described by which hydroid colonies, each on a microscopic slide, were raised from individual planulae of the leptomedusa known as Phialidium gregarium. 2. Growth and behavior of approximately 30 cultures were observed and quantitated through nearly 2 months. Cultures produced gonangia and medusae 3-7 weeks after fertilization of the egg. 3. Gonosome development was always preceded by a burst of hydroid development 5-9 days previously. This burst appeared to be initiated entirely by an abundant food supply (brine shrimp). 4. Medusae were liberated 4-5 days after gonangia first appeared. From the growth rate of medusae under the less than optimal laboratory conditions, it was estimated that sexual maturity may be reached in approximately 3 weeks. The lifespan of the medusae probably does not exceed 3 months. 5. A small number of cultures wintered in an open cage in the sea. At the end of November they were found in a greatly reduced state and possessed only a very few, very small hydranths. At the end of March they were in the early phases of vigorous trophosomal growth and the gonosome had just begun to flourish. 6. From the behavior of clonal colonies in the laboratory, the hypothesis was derived that the swarms of mature medusae in nature are the direct result of a "bloom" in zooplankton which occurred 5-7 weeks previously and stimulated first hydroid development and in consequence the formation of gonangia. 7. Morphological characteristics of the hydroid colonies were described and their variability under laboratory conditions recorded and discussed. It was shown that many taxonomic features such as size, branching, number of tentacles and annulations depend quantitatively on the age of the individual colony, its state of nutrition and on genetic factors. 8. The hydroid belongs to the genus Clytia (Lamouroux, 1816). A careful comparison was made with species of the genus as described for the Puget Sound region and California. It was concluded that none of these delineates unambiguously the morphological features of this hydroid. A tentative name for the hydroid was assigned according to priority principles as Clytia gregaria. It was suggested that Phialidium gregarium may be the most appropriate name for the species in both its phases.