Taxonomy, phylogeny and host-parasite interaction of two novel Myxobolus species infecting Brycon orthotaenia from the São Francisco River, Brazil.  Two new Myxobolus species were described infecting Brycon orthotaenia from the São Francisco River, in the state of Minas Gerais, Brazil. From a total of 39 B. orthotaenia collected, two specimens (5.1%) exhibited infection of the ovary and 12 specimens (30.8%) displayed infection of the liver. The plasmodia of both Myxobolus species were white and spherical measuring around 1 mm in length. The plasmodium found in the ovary showed mature myxospores, which were oval shaped from the frontal view and measured 9.2-11.0 (9.8 ± 0.4) μm in length, 5.9-6.9 (6.5 ± 0.3) μm in width and 4.6-5 (4.9 ± 0.1) μm in diameter. The two polar capsules were the same size and measured 3.9-6.2 (4.7 ± 0.5) μm in length and 1.8-2.4 (2.1 ± 0.2) μm in width. The polar tubules had 9 coils. The plasmodium found in the liver showed mature myxospores which were ellipsoidal in shape from the frontal view and measured 10.0-11.4 (10.7 ± 0.5) μm in length, 7.3-8.6 (8.1 ± 0.4) μm in width and 5.3-7.0 (6.8 ± 0.4) μm in diameter. The two polar capsules were the same size and measured 4.2-5.4 (4.9 ± 0.3) μm in length and 1.9-2.9 (2.7 ± 0.3) μm in width. The polar tubules had 8 coils. Ultrastructural analysis revealed an asynchronous sporogenesis process, with young developmental myxospore stages more often found in the periphery of the plasmodium and mature myxospores in the centre of the plasmodium. The plasmodial wall was formed by a single membrane which was not surrounded by a layer of host tissue. A thick layer of fibrous material was found in the peripheral ectoplasm close to the plasmodial wall of the plasmodium found in the ovary. Phylogenetic analysis based on the small-subunit ribosomal DNA - ssrDNA sequences and using the closest myxozoan sequences to each one of the species studied here based on previous GenBank data and Henneguya/Myxobolus/Thelohanellus species parasitizing fish from South American, revealed that the new species are grouped in a subclade together with other Myxobolus species parasitizing bryconid hosts.