During a 6 mo study of moribund trout from Buford hatchery, Buford, Georgia, USA, a Loma cf. salmonae microsporidian parasite was studied in the gills of brook trout Salvelinus fontinalis, brown trout Salmo trutta, and rainbow trout Oncorhynchus mykiss. This parasite was morphologically similar to L. salmonae and L. fontinalis but differed in spore size. Scanning and transmission electron microscopy demonstrated that xenomas were embedded in gill filaments. Transmission electron micrographs prepared from fresh tissue showed mature spores with 12 to 15 turns of their polar tube. Spore diameters for the Georgia strain from formalin-fixed gill tissues measured 3.5 (SD ±0.1) by 1.8 (SD ±0.1) µm. Electron micrographs of formalin-fixed, deparaffinized tissues of rainbow trout from Pennsylvania and West Virginia show spores with a diameter of 3.5 (±0.2) by 1.7 (±0.1) µm and 3.4 (±0.2) by 1.8 (±0.1) µm, respectively. Transmission electron micrographs of spores from Pennsylvania and West Virginia show that mature spores from both states had 13 to 15 turns of their polar tubes. Measurements from transmission electron micrographs prepared from alcohol-fixed tissues from Virginia fish contained spores with a diameter of 3.0 (±0.3) by 1.1 (±0.3) µm and 12 to 15 turns of their polar tubes. These measurements are consistent with L. salmonae and therefore suggest that the parasite is present on the east coast of the United States. During the height of the Georgia epizootic, the percentage of fish with observed xenomas reached 62.2% (N = 87), and the highest number of xenomas counted per 10 gill filaments was 133 (N = 87). The microsporidian epizootic occurred either during the autumn months or when intake river water quality reached combined iron-manganese concentrations as high as 1.01 (mean 0.44, SD ±0.42) mg^-1.