Five postembryonic stages of the parasitic copepod C. medusaeus (Wilson) were recovered from the mantle cavities of 7 spp. of pelagic gastropods [Cavolinia tridentata, Clio balantium, C. cuspidata, C. pyramidata, Cuvierina columnella, Diacria trispinosa and Carinaria japonica] collected off southern California. Adult parasites and several transforming specimens were recovered from 7 genera of lanternfishes (Myctophidae) [Ceratoscopelus, Diaphus, Diogenichthys, Lampanyctus, Parvalux, Stenobrachius, Symbolophorus, Tarletonbeania and Triphoturus] from the same locality. Recovery of both larvae and adults permitted a detailed study of the life history of C. medusaeus. After hatching, free-swimming copepodids enter the mantle cavity of a gastropod intermediate host and anchor to the mantle tissue by means of a frontal filament. Development through 3 chalimus stages and copulation between adult males and preadult females occur in the intermediate host. After copulation, adult males die and only postmated females leave the gastropod and enter a fish host. Differential growth of the parasite''s body results in the development of an attachment organ grows into the host''s bulbus arteriosus and a trunk which penetrates the host''s body wall at the isthmus. The developmental stages from gastropod hosts collected off California are compared to the larvae of Cardiodectes sp. found in Janthina globosa Swainson from the West Indies. All known host and locality records for C. medusaeus are included.