The gummy shark (Mustelus antarcticus) is a major target species in two substantial shark fisheries that operate in temperate Australian waters. Data on the reproductive biology of M. antarcticus in the waters off south-western Australia were obtained from samples collected by commercial vessels operating from Albany to Esperance, Western Australia. The samples contained a ratio of four females to one male. M. antarcticus is a viviparous, aplacental species. Males mature at a smaller size than females. Since the overwhelming majority of sharks sampled were mature, it was not possible to estimate precisely the mean size at which sexual maturity was attained. Examination of seasonal changes in the development of ova and testes, in uterine contents, and in embryo growth established that the period of parturition, mating and ovulation occurred over the 3 months between early November and early February. The gestation period was 11-12 months. Full-term embryos ranged in size from 30 to 36 cm total length and occurred in a sex ratio of one male to one female. The ovarian and gestational cycles proceed concurrently, with reproduction occurring annually. Only one of the 224 females analysed for uterine content was considered to be in a true post-partum condition. The number of embryos (N) per mother increased with the length of the mother (L) according to the regression N = exp(-4.13398 + 0.049171L). The reproductive biology of females collected off Albany and Esperance differed in some respects from that of females collected off south-eastern Australia.