Temperatures were measured with fine wire, copper/constantine, thermocouples insulated withlacquer.The e.m.f.was measuredagainsta reference bathofice and waterinan insulated Thermos flaskat0°C. The recordinginstrumentwas a portablepotentiometer manufacturedby Pye InstrumentsLtd.,Cambridge, England. An accuracyof ±25°F. isclaimedby themanufacturer.Wet and drybulbairtemperatures weren-measured witha slingpsychrometer. Three species were seiected for study: the barnacle, Tetraclita squaniosa, the limpet, Fissureila barbadensis and the gastropod, Nerita tesselata. These species were abundant at the site chosen, a flat platform of beach rock which is fully ex posed at low tide. Body temperatures were measured by thrusting the thermo coupie tip under the operculum of Nerita, through the apical hole of Fissurella and between the plates of the carapace of Tetraclita. Temperatures of a black body and an inanimate body were also measured. The black body was a piece of soft black mastic and the inanimate body, the shell of a Nerita, stuffed with mastic. The deflection ofthegaivonometerwas notedimniediately, at-md eachanimalwas used onlyfora singletemperature measurement.