Abstract: The objective of the study was to ascertain the effects of high temperature stress on root growth of cotton genotypes viz. Qalandari, MNH-93, Rehmani, 5-12, CIM-70, SLH-41, MS-84, K-68-9, CRIS-6 and CiM-109. The seeds were germinated for 48h at 25+0.5°C. After 48 h they were heat stressed for 2 h at 25 (internal control), 30, 35, 37.5, 40, 42.5 and 45°C in a waterbath and re-incubated at 25°C for a further 22 h, making a total germination period of 72 h. The root growth in all cultivars showed a good tolerance upto 35°C. At 45°C the roots of all cultivars were irreversibly damaged. Between 35 and 45°C the cultivars behaved differently, however. Cultivars Qalandari and Rehmani were more heat tolerant, and cv. 5.12 least heat tolerant.