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Native and introduced fish can serve as reservoirs for pathogens of cultured fish species. In the current study, 351 archived western mosquitofish Gambusia affinis collected from experimental catfish production ponds in Mississippi, USA, were surveyed histologically to evaluate their potential as vectors for fish pathogens. In addition to epitheliocystis and multiple metazoan parasites, 8 fish had widespread basophilic colonies of small Gram-positive rods associated primarily with stroma supporting the skeletal muscle and bone, as well as connective tissue components of other tissues and organ systems, such as perivascular adventitia and basement membranes. These findings were consistent with spaC-type Erysipelothrix sp. infections in ornamental fish cultured in the USA. The 16S rRNA, gyrase B (gyrB), and surface protective antigen (spa) genes were amplified and sequenced from bacterial colonies excised from paraffin-embedded tissue sections using laser capture microdissection. Molecular data confirmed the identity of a spaC-type Erysipelothrix sp., which grouped phylogenetically with spaC-type Erysipelothrix sp. from diseased ornamental fish. Given the significance of commercial catfish aquaculture in the southeastern USA and the widespread distribution of mosquitofish in catfish ponds throughout the region, infectivity trials with channel catfish Ictalurus punctatus were conducted. Catfish fingerlings were exposed to a spaC-type Erysipelothrix sp. isolate by intracoelomic injection and gavage. No mortality was observed in catfish exposed by either route, and surviving fish demonstrated no significant histopathologic lesions, suggesting channel catfish have low susceptibility to the bacteria. Further research is warranted to investigate the susceptibility of other cultured fish species to this emergent fish pathogen.