These data describe behavioral and physiological responses to cooling in intertidal Phyllaplysia taylori sea hares. Critical thermal minima (CTmin) were assessed using behavioral response time (seconds) after stimuli to the rhinophores under a cooling temperature ramp of -0.4°C per minute. Critical thermal minima are reached when an organism loses homeostasis due to temperature, but can regain normal function after the experience with a recovery period. These behavioral and physiological responses were assessed in two populations from habitats that are disparate in environmental conditions (temperature regime, estuary size, flushing rate), and at four laboratory acclimation regimes representing a fully crossed temperature x salinity design reflecting the two wild habitats (11C & 17C; 22psu & 32psu). The two habitats were Tomales Bay, CA, USA and Humboldt Bay, CA, USA; individuals were collected and assessed in the laboratory in February-March 2018.