The male sterility-fertility response in thermosensitive genic male sterile (TGMS) lines was studied to identify their adaptability to different temperature regimes. Twenty suspected TGMS lines available at the Paddy Breeding Station, Coimbatore, were used for the study. The plants of these lines, which were completely male sterile during summer 1995, were tagged and observed for their male sterility-fertility behavior in the ratooned tillers during winter 1995 and summer 1996 by staining pollen grains with I-KI. Panicle development stages, from the formation of pollen mother cells to the late uninucleate stage of pollen grains, were considered as sensitive to temperature. These stages occurred 9-19 d prior to heading (or 9-17 d after panicle initiation). Therefore, the critical temperature at which plants showed variation in pollen sterility and fertility was calculated by considering the average daily temperature and average maximum temperature prevalent during 9-19 d prior to heading.