The temperature tolerances of three species of marine fishes----Menidia menidia (common silverside), Pseudopleuronectes americanus (winter flounder), and Spheroides maculatus (northern swellfish)-have been determined. Following calculation of the median tolerance limit for various exposure times over a range of acclimation temperatures, the thermal tolerance was determined on a square graph by plotting lethal temperature against acclimation temperature. The tolerances determined are: 715°C squared for M. menidia; 635°C or 685°C squared for P. americanus, with cold tolerance limits of 1°C or 0°C, respectively; and 550°C squared for S. maculatus.