The Catalina Conservancy Divers is a support group of the Santa Catalina Island Conservancy, a private, non-profit conservation organization dedicated to the preservation of the natural heritage of Santa Catalina Island. Personnel of this group in conjunction with the Wrigley Marine Science Center maintain a Ryan thermograph in Isthmus Cove, just outside the mouth of Fisherman Cove, on the northeast side of the island. The data presented here are the daily means of these subsurface (4 M) temperatures recorded every 30 minutes. Historically, temperature readings at Santa Catalina Island were measured manually in the surface waters at Isthmus Cove (1955-56, 1970-85) and Avalon Bay (1955-56, 1963-67). These measurements are therefore not comparable to the thermister data recorded at its current depth and location at the mouth of Fisherman Cove (1991-96).