Tests were conducted to determine the upper incipient lethal temperature and critical thermal maximum (CTM) for mosquitofish acclimated to 25.0 C and 35.0 C, the effects of various cyclic thermal acclimation regimes on the CTM, and the effects of severe daily heat stress on the CTM. Heat tolerance of mosquitofish subjected to brief daily exposure to temperatures approximating or exceeding the upper lethal limit was increased over that obtained with constant temperature acclimation in proportion to the peak temperature of the cycle.