In the Lazarev Sea, Southern Ocean, I studied the population structure and reproduction of Eukrohnia bathypelagica and E. bathyantarctica during winter and summer. A lack of unimodality in their population structures indicated at least two generations building one population and a life cycle longer than 1 year. During both seasons all maturity stages were present; therefore, continuous reproduction is very probable. Extensive breeding seasons with several releases of eggs by one generation are assumed, as adults with empty marsupial sacs continue to build new ova. E. bathypelagica carried between 86 and 128 eggs in both marsupial sacs. E. bathyantarctica had only between 8 and 13 eggs. Although self-fertilization seemed at least to be possible in E. bathyantarctica, cross-fertilization appears to be more common in both species, as most individuals developed testes and ovaries consecutively. © Springer-Verlag 2009.