Seasonality in the reproduction of Ekmocucumis steineni and Psolus dubiosus (Holothuroidea) from the Antarctic shelf was investigated. The material was collected in spring (October/November) and autumn (mainly February) in the southeastern Weddell Sea. For E. steineni differences between both seasons in relative female fecundity and relative gonad weight were found in both sexes. Large yolky oocytes (0.3-1.0 mm in diameter) forming 1-3 cohorts were present in spring, nearly none in autumn. A delayed or prolonged spawning is possible, depending on the locality. No seasonal differences were found in P. dubiosus, a brood-protecting species with a lower fecundity and larger eggs (≤1.5 mm in diameter). A longer spawning phase in P. dubiosus than in E. steineni (October to January) is suggested. Both species are suspension feeders and their intestines were more filled with organic material in autumn than in spring. However, the occurrence of mature oocytes was not directly linked with the seasonal availability of food. The size-frequency distribution of the entire material in the study area was heterogeneous, indicating differences in long-term environmental conditions. The data show an evolutionarily more evolved high Antarctic mode of reproduction in P. dubiosus with a prolonged reproductive period than in E. steineni which has a distinct annual spawning cycle. © 1992 Springer-Verlag.