The most significant events of the reproductive cycle of the Patagonian stone crab, Platyxanthus patagonicus, were studied in specimens sampled in northern Patagonian gulfs from August 2004 to May 2006. Male and female crabs were assigned to stages of a scale of gonad maturity. Females were also classified according to the presence/absence of embryos on the pleopods and of spermatic content in the spermathecae. Ogives were fitted to maturity at size data. The highest frequency of pre-ovigerous females and spermathecae with hardened seminal contents occurred during the fall and winter. Fifty percent of the females reached ovarian maturity at 66.4 mm CW. Ovigerous females were frequent from mid-fall to mid-spring, peaking during early winter and mid-spring, and were unusual or absent during the summer months. Post-ovigerous females were present in samples in all months but May and June 2005, being frequent from late-spring to early-fall. Specimens with spent ovaries were common in all seasons but summer, while those with mature or recovering ovaries were present in samples taken in all months but August 2004 and May-June 2005. Fifty percent of the males reached testes maturity at 54.7 mm CW. Males larger than this size showed mature gonads year round, but a pulse of individuals with recovering testes was observed during the winter of 2005 and early-fall of 2006. Our results show that mating occurs during the fall months, spawning and embryonic incubation extends from fall to early spring, and hatching takes place during late-spring and early-summer.