The histological and ultrastructural characteristics of a new species of Henneguya and the host reactions to infection by this species are reported. Henneguya caudalongula
sp. n. was found in the inter and intralamellar regions of the gills of Prochilodus lineatus
(Valenciennes, 1836) cultivated at Center for the Research and Management of Continental Fishing Resources located in the municipality of Pirassununga, state of São Paulo, Brazil. The plasmodia were white and round or ellipsoidal and measured 0.2 to 1 mm in length. The development of the parasite was asynchronous and the mature spores were fusiform, with a total length 71 ± 1.4 μm, body length of 16.6 ± 0.54 μm and width 4.6 ± 0.2 μm. The caudal process was 52.6 ± 1.5 μm long. The polar capsules were elongate (length 6.1 ± 0.19 μm, width 1.6 ± 0.15 μm) and of equal size. The polar filament was coiled in 10-11 turns. The prevalence of the parasite was 48.3% and did not vary significantly with the season or host size.