During March 2024, 2025, and 2026, 27 largescale stonerollers, Campostoma oligolepis, were collected from the Black and White River drainages in Lawrence County, Arkansas, USA, and examined for myxosporean parasites. All fish had their gills, gallbladder, fins, integument, musculature, and other major organs examined. The gill of 1 of 27 (3.7%) C. oligolepis was infected with a new myxosporean, Myxobolus campostoma n. sp. Qualitative and quantitative morphological data were obtained from fresh myxospores. The plasmodia were orbicular to very broadly elliptoid. The myxospores were orbicular to broadly elliptoid and measured (L × W), 8.0 ± 0.4 × 7.0 ± 0.4 µm in valvular view. Thick myxospores were 5.1 ± 0.2 µm in sutural view, with a distinct sutural ridge. Two polar capsules were broadly pyriform, equal to subequal in size. The longer polar capsule was 3.7 ± 0.3 × 2.1 ± 0.2 µm, and 46% of the myxospore length, while the shorter polar capsule was 3.4 ± 0.3 × 2.1 ± 0.2 µm, and 42% of the myxospore length. A coiled polar tubule was not discernable, and myxospores lacked a mucous envelope and sutural markings. The new species is most similar morphologically to Myxobolus stanlii Iwanowicz, Iwanowicz, Howerth, Schill, Blazer, and Johnson, 2013, found in connective tissue and muscle of C. oligolepis from Alabama, USA, by possessing orbicular to broadly elliptoid myxospores, but differs in the size of myxospores and polar capsules. Molecular data consisted of a 1994 base pair sequence (GenBank Accession: PQ859550) of the partial small subunit (SSU) ribosomal RNA (rRNA) gene. Phylogenetic analysis grouped M. campostoma n. sp. with catostomid-infecting myxobolids from North America. This is the second time a myxosporean has been described from C. oligolepis, as well as being the first time it has been collected from an individual of the species from west of the Mississippi River, USA.
