The parasites of 45 Serrasalmus rhombeus (Linnaeus, 1776) were collected during the dry season in six floodplain lakes of the Solimões River. The prevalence was 73.3%, the mean intensity 20.5 and the mean abundance 15 crustaceans. Three species of crustaceans parasitized S. rhombeus. Two species of Copepoda infected the gill filaments, Ergasilus yumaricus Malta, 1995 and Myracetyma piraya Malta & Varella, 1993 and one of Branchiura infected the pectoral fin and gill cavity, Argulus chicomendesiMalta & Varella, 2000. Ergasilus yumaricus was the dominant species (92.3%) and the only one that showed significant differences in their abundance among the six lakes sampled. The fishes from Baixio Lake had the highest mean abundance (H = 11.2986, p = 0.04). There was a positive correlation between abundance and the total length of S. rhombeus. There was no correlation between the number...