Apseudes heroae Sieg, 1986 (Family Apseudidae) and Allotanais hirsutus (Beddard, 1886) (Family Tanaidae) are common Tanaidacea of the southern Magellanic region. The aim of the investigation is to elucidate the postmarsupial development of these tanaid species that differ in their biogeography. Population structures are analysed from size frequency data and from different postembryonic stages of specimens collected in the Atlantic entrance of the Beagle Channel and from the Atlantic continental slope to the south-east. The population of Apseudes heroae shows three age groups, the one of Allotanais hirsutus probably at least three. Some large specimens indicate that the latter sub-Antarctic species might reach an age of several years. Both species are probably gonochoristic, and protandric hermaphroditism was not observed. The fecundity of Apseudes heroae and Allotanais hirsutus was analysed. No significant correlation could be found between cephalothorax width and egg number or number of larvae. For both species hypothetical life cycles are reconstructed. As sub-Antarctic temperatures are low and seasonality is strong in the Magellanic region, it is possible that these species are reasonably adapted in reproduction as shown in some polar species of Isopoda and Cumacea.