Complex survivor curves of heated bacteria are interpreted to be composites of several convex survivor cutves that represent populations of different heat resistances in a single culture of bacteria.The variation in heat resistance appears to be physiological rather than genetic since subcultures of heat-resistant cells were no more heat resistant than the parent culture. Composite cures can appear to be nearly exponential.Results support a multiple-site hypothesis of thermal death. Tailing of survivor curves, with small numbers of cells surviving extended heating, wus frequently noted when curves were carried through 6-9 log cycles. Such tailing might be of practical importance because it would predict that small numbers of cells might survive much longer heating than would be predicted from D-values calculated from curves carried through 4-5 log cycles.