The influence of three types of dams with different degrees of river connectivity on the structure of fish communities along the Paraíba do Sul River was studied: total blockage (Dam 1); partial blockage allowed by a permanent lateral channel (Dam 2); and total blockage, but with a mechanism operating in the summer for fish passage (Dam 3). The tested hypothesis is that the degree of river connectivity influences the fish community. The largest differences in fish fauna were expected between the reservoir and downriver stretch in Dam 1 with total blockage; an intermediate difference in Dam 3 with total blockage but with a fish passage; and the least difference in Dam 2 with partial blockage. Fish were caught by gill nets between January and March 2010 and between January and February 2011 (wet seasons) in two zones: the reservoir and the downriver stretch from the dam. A total of 43 fish species was recorded, including eight non-native and two marine species. The 13 most abundant species (n > 100; frequency of occurrence >20%) occurred in all three stretches of the river. The community structure changed significantly between the reservoir and associated downriver stretch, with higher richness downriver compared to the reservoir zone. A trend for higher occurrence of migratory fishes (e.g. Pimelodus maculatus, Pimelodus fur, Leporinus copelandii and Prochilodus lineatus) was found in the downriver zone, suggesting the influence of dams on their upriver migration. The predictions were not fully matched. Although the most significant difference in the fish community structure between the reservoir and the downriver was found for Dam 1, the partial blockage in Dam 2 showed broader differences in the fish fauna than the total blockage in Dam 3 with the fish ladder, which may indicate that the latter is not a guarantee that species and genetic flux will be interchanged, since the water velocity may be a constraint to upriver fish migration.