On the basis of a sampling program performed monthly, a total of 12,033 fiddler crabs, Uca uruguayensis, were collected at the southern end of the Samborombón Bay, Río de la Plata estuary, Argentina, from February 1995 to March 1996. Density ranged from 133 to 207 ind/m² over the 13-mo study period. The overall proportion of male crabs was 0.62. For small crabs, sex ratio was 1:1, but as crab size increased, sex ratio first became female biased and then male biased. Crabs carried eggs from mid-spring to late summer. Prevalence of the parasite Leidya distorta varied between 5.24% and 17.8% throughout the study. Infested male crabs ≥10.5 mm carapace width, which represented only 5.9% of the crabs collected, housed 68.8% of the total parasites recovered. Cryptoniscus larvae were much more frequent among molting (soft) than intermolt (hard) crabs. The cryptonisci infesting intermolt (hard) crabs were aggregated; i.e., they showed a contagious distribution. Females of L. distorta were actively breeding in spring and summer. In autumn, reproductive activity gradually decreased, and by winter, all the adult female parasites showed empty marsupia. The growth of immature parasites appears to stop in winter and resume in spring. The size of the adult marsupial female parasite was positively correlated with that of the host. Because only 1.1% of the mature female crabs carried marsupial parasites, it is unlikely that L. distorta plays a regulatory role in the reproductive potential of the host population.