Although it is a breeding species in many parts of the Great Basin (Ryser 1985) and along the Columbia River (Thompson and Tabor 1981), the Caspian Tern (Sterna caspia) has been infrequently recorded in Idaho. Burleigh (1972) listed it as a uncommon, local summer visitant to the southern part of the state with one record for northern Idaho at Lewiston, Nez Perce Co. He thought it might breed in Idaho, but knew of no colonies. Davis (1935), Levy (1950), and Oring (1962) recorded it in small numbers from Rupert (Minidoka Co.), south-central Idaho, and Camas National Wildlife Refuge (Jefferson Co.), respectively. Larrison et al. (1967) stated that it was known to breed only at the Dingle Marsh in Bear Lake Co. but gave no details. I present evidence here that this tern breeds at several locations in southern Idaho, is sometimes found in moderate concentrations in this part of the state, and is more than an accidental visitor to the northern part of the state.