Myxidium ceccarellii n. sp. (Myxosporea) from the gallbladder of Leporinus elongatus (Anastomidae) from the São Francisco River, Brazil. During a survey of myxozoan parasites of freshwater fish from the São Francisco River in Minas Gerais State, Brazil, plasmodia of Myxidium ceccarellii n. sp. were found in gallbladders of two out of six specimens (22%) of Leporinus elongatus (Anastomidae). Parasite plasmodia were translucent and greenish, with disporic sporoblasts that develop asynchronously. Mature myxospores were ellipsoidal in frontal and lateral views, with slightly pointed ends. The surfaces of each valve had four to six longitudinal grooves. Spores dimensions were as follows: length 17.7 ± 0.5 μm (17.1-18.1), width 10.4 ± 0.47 μm (9.8-11.3), and thickness 10.1 ± 0.27 μm (9.6-10.4). Two polar capsules, one at either end of the spore, had the length of 6.3 ± 0.5 μm (5.7-7.0) and width of 6.4 ± 0.44 μm (5.7-6.9), with four to five polar filament turns. Some aberrant spores had one or three polar capsules. Partial sequencing of M. ceccarellii n. sp. small subunit ribosomal RNA (ssrRNA) gene resulted in 1,845 bp. This is the first molecular study of a Myxidium species that parasitizes a South American freshwater fish. Phylogenetic reconstruction using ssrRNA gene sequences showed that M. ceccarellii n. sp. was positioned basally in a recognized clade of myxozoans that infect the biliary systems of nonfish vertebrates.