Catchable-sized, hatchery-reared rainbow trout (Salmo gairdneri) undergoing survival tests in controlled sections of a mountain stream repeatedly declined in coefficient of condition for several months after being stocked. Examples of the extent of this decline, together with records of stream temperatures and associated mortality are used to demonstrate the relationship among poor body condition, rising temperature, and breakdown of trout vitality during the critical late-winter period. Possible advantages of fall stocking and of breeding some hatchery trout for superior adaptability are discussed.