A new microsporidian, Pleistophora beebei sp. nov., parasitizing the freshwater benthopelagic teleostean fish Brachyhypopomus beebei Schultz, 1944 (fam. Hypopomidae) collected from the Amazon River is described based on molecular and morphological studies. The parasite develops in the skeletal muscle of the abdominal cavity, forming a whitish cyst-like containing several groups of two types of spores (macrospores and microspores), which were observed in close contact with the myofibrils. Small groups of macrospores (ovoid elongate, tapering more anteriorly than posteriorly and measuring about 7.8 ± 0.4 × 4.7 ± 0.2 µm) were observed among the numerous microspores (lightly pyriform to ellipsoidal with rounded ends, measured about 4.7 ± 0.3 × 2.8 ± 0.4 µm). Both types of spores possessed a single large posterior vacuole containing flocculent material. The ultrastructural aspects observed, together with the formation of a cyst-like, suggest that the parasite belongs to the genus Pleistophora. This taxonomic positioning was confirmed by the molecular analysis of the SSU rRNA gene and Maximum-likelihood (ML) inference. Comparison to similar species previously described, recognized this as a new species, herein named Pleistophora beebei sp. nov.
