1. On the coast of Florida subtropical lamellibranchs exposed in the tidal region demonstrate a cellular heat resistance which is about 6-7° C higher than in boreal species on the german coast of the North Sea. 2. Geographically separated populations of the same species, (Crassostrea virginica from Cape Cod and Florida, Mytilus edulis and Macoma balthica from both sides of the Atlantic), show only small differences of their cellular heat resistance. Thus, no genetically fixed distinctions may be recognized. 3. Changes of the temperature of pretreatment within the range 10-23.5° C cause the subtropical-tropical bivalve Chione cancellata to perform a "meaningful" adaptation of cellular heat resistance. 4. After pretreatment with a heat shock of isolated gill tissues a small increase of heat resistance is connected with an augmented resistance to acid.