Heat and cold tolerance was examined in the terrestrial slug, Lehmannia valentiana, in Osaka, Japan. In the field, both the heat and cold tolerance of slugs changes seasonally. Heat tolerance was maximal in summer and minimal in winter, whereas cold tolerance was maximal in winter and minimal in summer. To clarify the environmental factors by which temperature tolerance is affected, the effects of acclimation temperature and photoperiod were also examined in slugs hatched under laboratory conditions. Heat tolerance was enhanced by a higher acclimation temperature. Long-day conditions also increased heat tolerance. Cold tolerance was enhanced by both short-day conditions and a low acclimation temperature. These results indicate that seasonal changes of heat and cold tolerance are promoted not only by acclimation to ambient temperature but also by the photoperiod.