Morphology and molecular data (rDNA; internal transcribed spacer 2 [ITS2]) confirmed that Proterometra catenaria Smith, 1934, undergoes asexual reproduction in rasp elimia, Elimia floridensis, (Cerithioidea: Pleuroceridae) and that it sheds a furcocystocercous cercaria that matures in the epithelia of the buccal cavity, near the esophageal sphincter, of redspotted sunfish, Lepomis miniatus, (Perciformes: Centrarchidae) and longear sunfish, Lepomis megalotis, in Holmes Creek (Choctawhatchee River), Florida, U.S.A. Adults of P. catenaria differ from those of congeners by having an oral sucker ≥2.4× ventral sucker length, a pharynx ventral to the posterior musculature of the oral sucker, a short, robust (thick-walled) pars prostatica, an ovary abutting the ventral sucker, a uterus lacking convolutions between the ovary and ventral sucker and that extends beyond the lateral margins of the forebody ceca, and paired, pretesticular vitelline fields that rarely extend beyond the posterior margin of the ventral sucker. Cercariae of P. catenaria differ from those of congeners by having an elongate body (= tail stem furcae) 9–16 mm long (7 mm in fixed specimens); paired, ovate furcae that are longer than wide and taper posteriorly; 2 distinct anterior fields of tail stem mamillae; and a distome located in the middle of the tail stem. A preliminary phylogenetic analysis, including ITS2 sequences of P. catenaria plus Proterometra ariasae Womble and Bullard, 2015, Proterometra epholkos Womble, Orélis-Ribiero, and Bullard, 2015, Azygia longa (Leidy, 1851) Manter, 1926, and Leuceruthrus micropteri Marshall and Gilbert, 1905 (type species) had low support but did not reject assignment of P. catenaria to Proterometra, conspecificity with the azygiid adults and cercariae from Holmes Creek, monophyly of Proterometra, or the sister-taxa relationship between azygiids (L. micropteri and Proterometra spp.) that infect North American basses and sunfishes (Centrarchidae). The redspotted sunfish is a new host record for Proterometra. We also clarify the taxonomic identity of prosobranchs previously reported as hosts for P. catenaria.