In a parasitological survey of the least killifish, Heterandria formosa (Girard), from Ward Creek in Louisiana, we encountered a myxozoan parasite in the gallbladders of this host bearing myxospores consistent with a Myxidium species. Prevalence was 72.2% in 18 hosts examined, with infection characterized by the presence of sheetlike to discoid plasmodia, approximately 1–2 mm across, floating freely in the gallbladder. Myxospores were observed in the plasmodia and found floating freely in the bile and were elliptoid and fusiform in valvular and sutural views respectively. The myxospore poles were slightly pointed away from the midline of the sutural plane of the myxospore. The myxospore valve surface had 9–12 striations. Mean myxospore dimensions (range, µm) were 13.7 (13.0–14.2) long, 6.5 (5.4–7.2) wide, and 6.4 (6.1–6.7) thick. Two equal-sized polar capsules were broadly ovoid to spherical, oriented to opposite extremes of the myxospore, and contained a tubule with 4–6 coils. Mean polar capsule dimensions (and range) were 4.3 (3.6–4.9) long and 3.6 (3.2–4.4) wide. Morphologically, the myxospores of this species were distinct from those of most other Myxidium species, and for the few species where dimensions overlapped, the average dimensions were different, and host and locality were also distinct. As such, we described this as Myxidium fonti n. sp. in honor of William F. Font. Phylogenetically, M. fonti is sister to Myxidium phyllium from another poeciliid host, and these species fall into a larger biliary tract lineage of myxosporeans. The relationships support previous work that suggests that tissue specificity, with some exceptions, is important in the phylogeny of myxosporeans.