Thermal relations of the Indian leech,Poecilobdella viridis have been investigated. Leeches maintained at laboratory conditions (25·5°–27·5°C) were observed to have 36·65°C and 3·7°C as an upper and a lower lethal temperature respectively for 24 hr. Their 24 hr median heat tolerating capacity was elevated by 4·87°C after warm (32·0°±0·5°C) acclimation for 15 days. Ecophysiological significance of these changes toP. viridis in combating the ambient thermal fluxes has been hinted briefly.