Animal populations have a certain synchronization between their fluctuations and the environmental factors which have a direct influence on density, sex ratio and recruitment. Trichodactylus borellianus is a common freshwater crab of the Trichodactylidae family, originary from South America. This crab is omnivorous and grazes upon the littoral communities. The present study is the first analysis of the natural distribution and population dynamics of freshwater crabs in La Plata Basin (Argentina) and their relationship with certain abiotic and biotic parameters. It was carried out in three environments (riverine zone of streams and river) of the Paraná River floodplain. Crab populations were sampled monthly from August 2001 to October 2002. The mean crab abundance among the study sites was significantly different, being lower in samples from the environment surrounding the principal channel of the Paraná River. Crab density showed two peaks: the highest occurred during late winter and early spring, and the lowest during late summer. Males were only more abundant in late winter and spring. In spring, summer and autumn, the population showed a contagious distribution. The population displayed a regular arrangement in winter. From mid-spring to mid-autumn juveniles were very frequent whereas adults predominated the rest of the year. Total crab number in the stream nearest to the main channel varied according to the level and temperature of the river. In the other sites, located further from the main channel of the Paraná River, crab numbers and vegetation biomass showed a more similar arrangement with a higher correlation.