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+0.5EC. After 48 h they were heat stressed for 2 h at 25 (internal control), 30, 35, 37.5, 40, 42.5 and 45EC in a waterbath and re-incubated at 25EC for a further 22 h, making a total germination period of 72 h. The root growth in all cultivars showed a good tolerance upto 35EC. At 45EC the roots of all cultivars were irreversibly damaged. Between 35 and 45EC the cultivars behaved differently, however. Cultivars Qalandari and Rehmani were more heat tolerant, and cv. 5.12 least heat tolerant.