Morphological, ultrastructural, and phylogenetic analysis of two novel Myxobolus species (Cnidaria: Myxosporea) parasitizing bryconid fish from São Francisco River, Brazil.  Twelve Myxobolus species have been previously described to parasitize Bryconidae fish in South America. Here, we describe two novel myxosporean species that parasitize economically important Bryconidae from the São Francisco River basin in Brazil. Myxospores morphometry, morphology, small-subunit ribosomal DNA - ssrDNA sequences, and other biological traits were used in the taxonomic analysis. Phylogenetic analysis was performed to assess the position of the new Myxobolus species among the closest Myxobolus/Henneguya. Myxobolus iecoris n. sp. was found infecting the liver of Salminus franciscanus (dourado). Myxospores were oval with the anterior region aculiform in frontal view and biconvex in lateral view and measured 11.4-14.2 (12.8 ± 0.8) μm long, 7.7-9.9 (8.7 ± 0.6) μm wide, 6.5-7.5 (6.9 ± 0.4) μm thick. Two pyriform and equal-sized polar capsules measuring 4.9-7.4 (5.9 ± 0.5) μm long and 2.3-3.5 (3.0 ± 0.2) μm wide contained polar tubules with 8-9 turns. Myxobolus lienis n. sp. was found infecting the spleen of Brycon orthotaenia (matrinxã). Myxospores were round to oval in frontal view and biconvex in lateral view and measured 10.3-13.8 (12 ± 0.6) μm long, 6.8-9.3 (8.3 ± 0.5) μm wide, and 6.9-7.0 (7.0 ± 0.6) μm thick. Two oval and equal-sized polar capsules measured 3.9-5.8 (4.6 ± 0.5) μm long and 2.0-3.5 (2.8 ± 0.3) μm wide contained polar tubules with 5-6 turns. Ultrastructural analysis revealed asynchronous sporogenesis with germinative cells and young sporogonic stages in the periphery of the plasmodia. A connective tissue capsule was observed surrounding Myxobolus lienis n. sp., but it was absent for Myxobolus iecoris n. sp. Maximum likelihood and Bayesian inferences showed the two novel species clustering in a well-supported subclade composed by Myxobolus spp. of bryconids. Myxobolus iecoris n. sp. appeared as a sister species of M. aureus and Myxobolus lienis n. sp. as sister to M. umidus.