A new ceratomyxid parasite was examined for taxonomic identification, upon being found infecting the gall bladder of Hemiodus microlepis
(Teleostei: Hemiodontidae), a freshwater teleost collected from the Amazon River, Brazil. Light and transmission electron microscopy revealed elongated crescent-shaped spores constituted by two asymmetrical shell valves united along a straight sutural line, each possessing a lateral projection. The spores body measured 5.2 ± 0.4 μm (n = 25) in length and 35.5 ± 0.9 μm (n = 25) in total thickness. The lateral projections were asymmetric, one measuring 18.1 ± 0.5 μm (n = 25) in thickness and the other measuring 17.5 ± 0.5 μm (n = 25) in thickness. Two equal-sized subspherical polar capsules measuring 2.2 ± 0.3 μm in diameter were located at the same level, each possessing a polar filament with 5-6 coils. The sporoplasm was binucleate. Considering the morphometric data analyzed from the microscopic observations, as well as the host species and its geographical location, this paper describes a new myxosporean species, herein named Ceratomyxa microlepis
sp. nov.; therefore representing the first description of a freshwater ceratomyxid from the South American region.