The life history of the hydrozoan, Hybocodon prolifer, was observed. The polyp from the actinula larva forms the stolon soon after metamorphosis. The stolon is dormant during the warm seawater season. The polyp does not produce the medusa. In early winter the stolon starts to form the polyp, and the polyp produces the medusae. Subsequently the polyp forms the stolon. Experimentally the stolon can be induced to form the polyp at any time during aestivation by treatment with low temperature seawater. Neither the polyp from the actinula nor that from the aestivating stolon forms the next polyp. In this case the hydrozoan is solitary. When the polyp forms the stolon with branches, a polyp is formed from each branch after aestivation. In this case the hydrozoan is colonial. If the stolon piece is divided into several pieces, a polyp is formed from each piece. When the contents of the stolon are extruded, the extruded cells form a cellular mass in several hours, and undergo morphogenesis to form a polyp, as does an intact stolon. The stolon is covered by a thick perisarc, and the endodermal cells contain large quantities of yolk-like granules. A few nematoblasts are found dispersed among the endodermal cells.