Fiddler crabs Uca uruguayensis were collected monthly in Punta Rasa, at the southern end of the Samborombón Bay, Río de la Plata, Argentina, from February, 1995, to March, 1996. Throughout the study, 1,115 of 12,033 crabs (9.3%) were infested by different developmental stages of the branchial bopyrid Leidya distorta, which is herein reported for the first time as infesting U. uruguayensis. The distribution of this parasite, previously known from New Jersey to Rio de Janeiro, is now extended even farther south, to the mouth of the Río de la Plata. Prevalence of L. distorta increased as a function of crab size. The newly settled cryptoniscus larvae were found in the space between contiguous gill lamellae after invading a previously uninfested branchial chamber. The cryptoniscus larva transformed into an early juvenile and, after molting one or more times, migrated to the roof of the branchial chamber, where it grew and finally reached the ovigerous condition. Female parasites usually carried cryptonisci or males, eventually both, on their bodies. Males were found in different positions on the females as the latter developed. Crabs housing a mature female parasite frequently bore an unpigmented area on the lateral wall of the branchial chamber. An additional faded area was sometimes observed on the bottom of the eye orbit. Adult female parasites usually produced a subtle lateral swelling on the carapace of the host, this deformation being more marked in smaller crabs than in larger ones. The major chela of the male crabs parasitized by adult females was, on average, 6.5% shorter than that of unparasitized ones. Alterations of the gills of the host were noticed: the adult female parasite had a strong dorsal carina that fitted between the fourth and fifth gills of the crab, displacing them laterally and making contact with the floor of the branchial chamber. The inner surface of the crab branchiostegite bore a large horizontal vessel from which the parasite presumably obtained hemolymph. Besides the bopyrid isopod forming the subject of this report, we found 61 crabs harboring a nematode larva, identified as Skrjabinoclava sp., among the internal organs of the crab.