Sanguinicola fontinalis sp. nov., from the branchial, renal and cardiac blood vessels of brook trout. Salvelinus fontinalis (Mitchill), and longnose dace, Rhinichthys cataractae (Valenciennes), is described. The new species most closely resembles, hut differs from, Sanguinicola davisi of steelhead and rainbow trout, Salmo gairdneri Richardson. The adult differs from all known North American species in having 29–33 lateral tufts of blunt spines. The snail host is Leptoxis (Mudalia) carinata (Bruquière, 1792). Lodged and migrating miracidia caused considerable gill damage in the brook trout, resulting in high mortality when the fish were moved. To remove the hazard, the most severely affected brook trout were destroyed and the ponds drained and disinfected. Adult worms from Pennsylvania brook trout and West Virginia longnose dace were studied, as well as infected snails.