The size at onset sexual maturity in brachyurans can be evaluated considering different criteria, including growth allometry, gonad development stages, and presence of sperm in spermathecas or eggs in the abdomen. Morphometric aspects of males and the presence of eggs in females of P. patagonicus from the northern San Matías Gulf were respectively analyzed to determine its size at onset of the sexual maturity. Crab samples were obtained during a fishing experiment with traps (cubical, pyramidal, and conical, of similar internal volume), performed in the gulf during winter 2005. Also, the parasitism by a sacculinid rhizocephalan was analyzed to discuss possible effects on the reproductive success. All crabs caught were sorted by sex, carapace width (CW) and total weight (TW). Size frequency distributions were compared among sexes. Proportion of females bearing eggs were grouped into 16 size intervals of 5 mm to determine the size of first reproductive maturity based on a criterion of L50%. In males, the allometric change of chela length was analyzed for this purpose. Only individuals exceeding 50 mm were caught. Of 807 crabs caught, 36% (289) were males and 64% (518) were females. The size frequency distributions differed significantly between sexes with males larger than females (K-S test). Relationships of total weight (TW) to carapace width (CW) were calculated for all reproductive types and compared by ANCOVA with the following results: parasitized crabs < females < females with eggs = males. The mean size at first maturity of females was estimated to be 87.66 mm. Size at maturity for male crabs could not be determined from morphometric data because there was no significant difference in the relationship of chela length and CW for juvenile and adult males. All parasitized crabs (73) were females, whose mean width and mean weight were significantly lower than those from nonparasitized individuals. Prevalence was 9% and the mean intensity was one parasite per host. The parasitized crabs resembled juveniles. The size at first maturity and the reproductive capacity of P. patagonicus could be affected by presence of the parasite, even producing significant interference at the population level. Parasitism by sacculinid must be monitored after the starting of an emerging fishery to assess the actual contribution of the reproductive stock.