Gastropods are described for the first time from the Kersadiou Formation (middle Givetian) in the vicinity of Brest, Brittany, northwestern France. Twenty-three species of gastropods are recognized. New taxa include the new genera, Calvibembexia, Kersadiella, Breizospira, Finisterella, Parahormotomina, and Nodoloxonema, represented by the new species Calvibembexia lethiersi, Kersadiella babini, Breizospira crozonensis, Finisterella tibidiensis, Parahormotomina sibertae, and Nodoloxonema plusquelleci. Further new species include: Sinuitina (Sinuitina) morzadeci, Crenistriella armoricana, Patellilabia (Phragmosphaera) ponceti, Angyomphalus (Eoangyomphalus) weyanti, Eoplatyzona ongaroae, Murchisonia oehlerti, and Hormotomina fiacrensis. The biogeographic affinities of the fauna show a strikingly unusual admixture of Eastern Americas Realm taxa such as Hormotomina and Crenistriella. This suggests that during the middle Givetian, the Massif Armoricain (northwestern France) represented an area of Realm-boundary mixing between the Old World and Eastern Americas Realms, while older Devonian gastropod faunas of the Massif Armoricain are wholly of Old World Realm type.