Twenty-seven specimens of cardinal tetra Paracheirodon axelrodi, 33 rosy tetra Hyphessobrycon copelandi (Characidae), 28 marbled hatchetfish Carnegiella strigata, 26 blackwing hatchetfish Carnegiella martae (Gasteropelecidae), 27 bodó Ancistrus hoplogenys (Loricariidae), 31 brown pencilfish Nannostomus eques, 38 oneline pencilfish Nannostomus unifasciatus (Lebiasinidae) and 13 angelfish Pterophyllum scalare (Cichlidae) were collected from the middle Negro River, State of Amazonas, Brazil, for parasitological studies. Out of the total of 223 fish examined, 143 (64.1%) were parasitized by at least one parasite species. The highest prevalence rate was for Monogenea (36.7%), followed by Ichthyophthirius multifiliis (Ciliophora) (20.6%), Trichodina spp. (Ciliophora) (4.0%), Piscinoodinium pillulare (Dinoflagellida) (1.3%), Tetrahymena sp. (Ciliophora) (0.89%), and Procamallanus sp. (Nematoda) (0.4%). All eight fish species had Monogenea (Gyrodactylidae and Dactylogyridae) in the gills, but the highest prevalence occurred in P. scalare and the lowest in P. axelrodi and C. strigata. However, the highest mean intensity of Monogenea was found in P. scalare and A. hoplogenys. The protozoan I. multifiliis occurred in the six ornamental fish species examined, but C. strigata and C. martae had higher prevalence and mean intensity. Trichodina spp. were found only in the gills of C. strigata, C. martae and N. eques, and with higher mean intensity in C. strigata. On the other hand, the protozoan P. pilullare was found only in the gills of C. martae. This is the first report of Tetrahymena sp. in Brazil, and it occurred in the gills of C. strigata.