Bartolius pierrei n. g., n. sp. (Digenea: Gymnophallidae) is described from metacercariae and naturally and cultivated obtained adults from southern Argentina. The second intermediate host is Darina solenoides (King) (Bivalvia: Mactridae) and the definitive host is Larus dominicanus Lichtenstein (Aves: Laridae). The diagnostic characters are as follows: Body small, oval. Oral sucker without lateral projections, twice size of ventral sucker (except in young metacercariae). Caeca short (in adults), without dorsal diverticula. Ventral sucker in posterior third of body. Ventral pit absent. Seminal vesicle bipartite. Ovary post-testicular. Vitelline glands paired, compact, close to ventral sucker. Uterus in fore- and hindbody. Genital atrium tubular. Genital pore inconspicuous, close to anterior margin of ventral sucker. Excretory vesicle Y-shaped with very short stem. Excretory formula: 2[(2+2)+(2+2)] = 16. Bartolius is distinguished from other genera of the Gymnophallidae by the post-testicular position of the ovary.