Temperature preference of yearling Colorado squawfish, Ptychocheilus lucius, was determined in a horizontal gradient trough. Fish were acclimated to 14, 20, and 26 C, and twenty fish were tested from each acclimation temperature. Acute preferenda were 21.9, 27.6, and 23.7 C for 14, 20, and 26 C -acclimated fish, respectively. Final preferendum was estimated as 25 C. The Colorado squawfish (Pytochocheilus lucius), endemic to the Colorado River drainage, is the largest native minnow in North America. Historically it was found throughout the Colorado River basin, primarily in the main- stream channels of the Colorado and Green rivers and in the large tributar- ies such as the Gila, San Juan, Gunnison, and Yampa (Behnke and Benson, 1980). Large dams, built in the 1930's through the 1960's, have presumably caused a rapid decline in Colorado squawfish abundance and distribution to the point it is now classified as an endangered species. The species is no longer found in the lower Colorado River basin, but is restricted to areas of the upper basin where detrimental effects of dams and of land- and water- use practices are less. Changes in the annual water temperature pattern imposed by these dams was cited as one possible cause for decline of this species. Temperature is recognized as an important in the life history of all fishes (Fry, 1947; Bartholomew, 1977). Brett (1971) considered temperature as the ecological master factor among the abiotic environmental factors. Giatinna and Gar- ton (1982) emphasized the importance of determining preferred temperatures of fish, not just lethal temperatures. Preferred temperatures should reflect thermal optima for certain biological processes. These processes may be functional over the entire range of normal habitat temperatures for a species, but would function best at or near the preferred temperature of that species. For a particular fish species, reduction of desirable habitat could occur through changes that effectively reduce the area of water having tempera- tures within the zone of thermal preference. The objectives of this study were to determine acute temperature preferen- ces of yearling Colorado squawfish, the influence of recent thermal history on preference, and to estimate first thermal preferendum for this life stage. Fish presented with a suitable range of temperatures in a restricted space will tend to congregate in, or spend most of the time in, a relatively narrow range of temperatures (Fry, 1947). Such behavior is termed temperature pref- erence or behavioral thermoregulation; and the temperature in which the congregation occurs, or in which the most time is spent, is the preferred temperature or thermal preferendum (Reynolds and Casterlin, 1979). Depending on the length of time spent in a thermal gradient, the preferred temperature might or might not be influenced by the recent thermal history of the fish. Acute temperature preference is usually determined within two