The life cycle of Diplostomum spathaceum (Rudolfi 1819), as it exists in North Park, Colorado, is elucidated. California gulls (Larus californicus), common mergansers (Mergus merganser) and Forster's terns (Sterna forsteri) act as definitive hosts. Two species of water snails, Physa gyrina and Lymneae elodes, act as first intermediate hosts with seven species of fish as second intermediate hosts: brassy minnows (Hybognathus hankinsoni), brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), mackinaw (Salvelinus namaycush), rainbow trout (Salmo gairdneri), western long nose suckers (Catostomus catostomus) and western white suckers (C. commersoni). Infection of fish by larvae of D. spathaceum results in cataracts of the lens and impaired vision.