These data describe behavioral and physiological responses to cooling in intertidal Phyllaplysia taylori sea hares. Reaction time was used as a behavioral response to temperature, and measured by physical stimulation of the rhinophores. The time (seconds) was measured from first stimulation to full re-extension of the rhinophores after disturbance. Rhinophores are sea hares' main sensory organs, and direct most of their interactions with their habitat, making their sensitivity important for ecological purposes. 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.