Two line breeding is a viable proposition in a state like Kerala where rice is cultivated from below mean sea level to altitudes of 1500 MSL. The system provides a much simpler and economic hybrid seed production and broader choice of male parents for enhancing yield potential. A set of seven promising Temperature Sensitive Genic Male Sterile lines received from Tamil Nadu Agricultural University, Coimbatore, India viz., TGMS 74S, TGMS 81S, TGMS 82S, TGMS 91S, TGMS 92S, TGMS 93S and TGMS 94S were characterized for their sterility, fertility behavior under field conditions. Based on sterility/fertility behavior the entire study period of 2011 March to 2012 March can be grouped as two distinct periods viz, pollen sterile and pollen fertile months. The pollen sterile months indicated April and May when 100 per cent sterility was observed. July and August can be considered as pollen fertile months. Other months exhibited un stable expression of sterility /fertility and hence could not be grouped under pollen sterile or pollen fertile months. The sterile period was longest in TGMS 82S (87 days) followed by TGMS 81S (83 days). Since all these lines showed completely sterile for more than 30 consecutive days during sterile phase, these lines were successfully utilized in hybrid rice breeding programme. The Critical Sterility Temperature ranged from 34.2⁰C (TGMS 74S, TGMS 91S, and TGMS 94S) to 38.6 ⁰C

(TGMS92S). Critical fertility temperature ranged from 23.6 ⁰C (TGMS 91S) to 25.4⁰C (TGMS 92S, TGMS 93S).