In Cnidaria, a separation between soma and germline remains unclear. In this work, we studied the origin of germinal cells and determination of the sexual phenotype in Clytia hemisphaerica and Clytia sp. Colonies of C. Hemisphaerica were cultivated and the medusae liberated by each colony raised until maturity. Two hermaphrodite colonies were obtained, liberating male and female medusae. These two colonies and their medusae were raised at 15 degrees C, 21 degrees C, or 24 degrees C. The medusae budded and cultured at 24 degrees C were mainly female (80%). In contrast, if the medusae were released at 15 degrees C, at whatever temperature they were raised later, they were mainly male (85%). The same occurred if, after release at 24 degrees C but before the formation of the gametes, they were kept at 15 degrees C for at least 24 hr. We suggest that there are two subpopulations of germ cells. The female line will be dominant at 24 degrees C but temperature sensitive, with inhibition of this line by a temperature drop to 15 degrees C, this inverting the population sex-ratio. The irreversible action of a temperature drop to 15 degrees C supports the view that the germ cells are isolated very early. In C. hemisphaerica, hermaphrodite medusae were never observed. On the contrary, in Clytia sp., probably a new species, we have found male, female, but also hermaphrodite specimens. This is the second definite example of hermaphroditism described in any hydromedusan. The transformation of female into hermaphrodite then into male specimens occurs at 13 degrees C. These results demonstrate the unstable character of genetic sex determination in cnidarians, at least in certain species.