It is well known that animals differ greatly in their tolerance of extreme temperatures, high or low, and that such differences are correlated with the temperatures of the organism's natural environment. With regard to a particular ecological group, the intertidal animals, it has been frequently stated that they are subjected to and adapted to tolerate much higher temperatures than related forms which are permanently submerged, and that within a particular area the resistance of a particular species is correlated with its position in the intertidal zone.Intertidal organisms live an amphibious life, being periodically in or out of the water. Since the amplitude of temperature changes in air greatly exceeds those in water, one would expect that one of the adaptations of a marine organism to periodic existence out of the water would be the ability to stand high temperatures. The magnitude of temperatures to be tolerated would then also depend on the climatic characteristics of a particular area. vVhile these principles have often been stated in recent reviews (GuNTER 1957, NEWELL 1964, UsHAKOV 1964, and their verity can hardly be doubted, underlying evidence is often sketchy, incomplete, or based on faulty experimentation. Most of the relevant data apply to relatively temperate zones, and are based on criteria which, as will be explained in the section on methods, are only useful in a comparative sense.In the voluminous recent Russian literature on this subject (see the comprehensive reviews by ALEXANDROV 1964, U SHAKOV 1964, and the collection of articles, Problems of Cytology, 1961) the adaptation of an organism to the temperature of its surrounding has been taken largely for granted, at least in all the work concerning intertidal molluscs, and the inquiry deals with the heat resistance of isolated muscles and enzymes. Some of this work, insofar as it is relevant to the present investigation, will be discussed later. v\'hile there now exists massive evidence for a correlation of muscular activity and enzyme action according to the temperatures to which intertidal molluscs in different climates and tidal zones are subjected, there still exists a gap in our knowledge concerning the tolerance of the whole organism to high temperatures. I began the analysis of the heat resistance of intertidal snails with what I considered 1)