Some biological parameters of the oyster (Crassostrea rhizopharae), during the spawning and post-spawning periods, were analised in relation to the infestation by the pea crab (Pinnotheres ostreum) The biological parameters considered were: size, total weight, wet and dry meat weight, and condition index. Utilizing the χ2 test these parameters and infestation by the pea crab were shown to to be associated. The incidence of pea crabs was higher during the spawning period than in the post-spawning period, reaching a maximum (20.3% infestation) in oysters with a size of 6 to 7 cm. The values of the condition index and percent of meat (wet and dry weight) for these oysters were significantly (P < 0.05) lower than for uninfested oysters. From these results there seems to be no doubt that the pea crab (V. ostreum) injures the mangrove oysters which supports the conclusion that these crabs are true parasites for C. rhizophorae.