Critical temperatures and the upper lethal limit (ULL) of field collected workers of Formosan subterranean, Coptotermes formosanus Shiraki, and eastern subterranean termites, Reticulitermes flavipes (Kollar), were measured monthly from September 2001 to August 2002. Monthly mean critical thermal maxima (CTMa), ULL, and critical thermal minimum (CTMin) fluctuated significantly throughout the year. Seasonal variations of CTMax and ULL of both species were slightly, but significantly, statistically correlated with seasonal variation of habitat soil temperatures at 15-cm depth. Regression analyses revealed a significant linear relationship between CTMin and habitat temperatures for both termites as well. The two termite species responded positively to temperature changes in a similar manner. The values of all three parameters were significantly greater for C. formosanus (CTMax: 44.8–45.9°C, ULL: 49.1–50.4°C, CTMin: 7.2–9.0°C) than R. flavipes (CTMax: 43.5–44.9°C, ULL: 46.9–48.3°C, CTMin: 1.0–4.9°C). R. flavipes had a wider range between the upper and lower critical temperatures than C. formosanus. The three thermal tolerance parameters were not correlated with worker body mass. The significance of the critical and lethal temperatures and comparisons with previous studies are discussed.