The class Myxosporea of the phylum Myxozoa contains 52 genera (Kent M.L., Andree K.B., Bartholomew J.L., ElMatbouli M., Desser S.S., Devlin R.H., Feist S.W., Hedrick R.P., Hoffmann R.W., Khattra J., Hallett S.L., Lester R.J.G., Longshaw M., Palenzeula O., Siddall M.E., Xiao C.X. 2001: J. Eukaryot. Microbiol. 48: 395–413), most of which parasitize fishes. Henneguya Thelohan, 1892 is the second most common of these genera and contains more than 150 species, some of which are important pathological agents (Dykova I., Lom J. 1978: J. Fish Biol. 12: 197–202; Kalavati C., Narasimhamurti C.C. 1985: Arch. Protistenkd. 129: 199–202; Lom J., Dykova I. 1995: In P.T.K. Woo (Ed.), Fish Diseases and Disorders. Protozoan and Metazoan Infections. Vol. 1. CAB International, Wallingford pp. 97–147; Martins M.L., Souza V.N., Moraes J.R.E., Moraes F.R. 1999: Rev. Bras. Biol. 59: 527– 534). In South America, Henneguya is the most abundant genus, with 31 species. To date, two species of Henneguya have been reported in Serrasalmus spp.: Henneguya iheringi Pinto, 1928 parasitizing Serrasalmus spilopleura caught in the state of Sao Paulo, and H. striolata Casal, Matos et Azevedo, 1997 parasitizing S. striolatus collected in the Amazon River estuary near Belem, state of Para, Brazil. Fish of the genus Serrasalmus are voracious carnivorous fish popularly known as piranhas and are widely distributed throughout South American rivers. In this study, we describe a new species of Henneguya parasitizing S. spilopleura. Eighteen adult and juvenile specimens of S. spilopleura were collected locally from a lake on a farm in the municipality of Campinas, state of Sao Paulo, Brazil, and examined for the presence of myxosporeans. The fish were captured between January and June 2001, and transported alive to the laboratory where they were killed by transection of the spinal cord then measured and necropsied. The measurements of 30 fresh mature spores (Lom J., Arthur J.R. 1989: J. Fish Dis. 12: 151–156) were obtained using a micrometer incorporated into a microscope eyepiece. The dimensions were expressed as the mean ± standard deviation (SD). India ink staining was used to detect the mucus envelope. The spores were checked for the presence of an iodinophilous vacuole after adding a drop of Lugol solution. Smears containing free spores were stained with Giemsa’s solution and mounted in low viscosity medium as permanent mounts (Adriano E.A., Arana S., Ceccarelli P.S., Cordeiro N.S. 2002: Folia Parasitol. 49: 259–262). For histological analysis, portions of the gills containing plasmodia were fixed in 10% buffered formalin for 24 h, embedded in paraffin, cut into sections 4 μm thick and stained with sirius red (Adriano et al. 2002, op. cit.), haematoxylin and eosin and PAS.