Three primates (Alouatta caraya, Sapajus nigritus and Sapajus cay) are found in riparian areas of the Upper Paraná River in southern Brazil. Population densities of these three species were estimated from October 2004 through September 2005 by counts on linear transects in riparian forests on two riverine islands and adjacent shores in the states of Mato Grosso do Sul and Paraná. A total of 397 sightings of these species were accumulated in 188 km of traversed transects. Alouatta caraya was the most abundant species on the large island (2.56 ind ha-1) and on the Mato Grosso do Sul side (0.84 ind ha^-1). This density was greater than twice that of the sympatric S. cay (0.31 ind ha^-1). Sapajus nigritus was the most abundant species on the Paraná side (0.51 ind ha^-1) followed by A. caraya (0.40 ind ha^-1). The folivorous A. caraya was the most abundant in flooded forests, while the omnivorous Sapajus species were less so. Differences in forests, conservation status, dispersal restrictions and autecology of the primates help explain differences in primate abundance.