It is generally accepted that crustaceans detect, and respond to, changes in water temperature, yet few studies have directly addressed their thermosensitivity. In this investigation a cardiac assay was used as an indicator that lobsters (Homarus americanus) sensed a change in temperature. The typical cardiac response of lobsters to a 1-min application of a thermal stimulus, either warmer (n = 19) or colder (n = 17) than the holding temperature of 15 degrees C, consisted of a short bradycardia (39.5 +/- 8.0 s) followed by a prolonged tachycardia (188.2 +/- 10.7 s). Lobsters exposed to a range of rates of temperature change (0.7, 1.4, 2.6, 5.0 degrees C/min) responded in a dose-dependent manner, with fewer lobsters responding at slower rates of temperature change. The location of temperature receptors could not be determined, but lesioning of the cardioregulatory nerves eliminated the cardiac response. Although the absolute detection threshold is not known, it is conservatively estimated that lobsters can detect temperature changes of greater than 1 degree C, and probably as small as 0.15 degrees C. A comparison of winter and summer lobsters, both held at 15 degrees C for more than 4 weeks, revealed that although their responses to temperature changes were similar, winter lobsters (n = 18) had a significantly lower baseline heart rate (34.8 +/- 4.4 bpm) and a shorter duration cardiac response (174 s) than summer lobsters (n = 18; 49.9 +/- 5.0 bpm, and 320 s respectively). This suggests that some temperature-independent seasonal modulation of cardiac activity may be occurring.