Divergent phenotypic selection was performed for cell membrane thermostability (CMS) in wheat (Triticum aestivum L.) in five F₂ populations derived from crosses established between eight local landraces quite variable in heat susceptibility index. CMS was assayed in the flag leaves of field-hardened segregating plants at anthesis. Selection was imposed on 200 F₂ spaced plants for each of the five populations where the highest and the lowest five plants in CMS were selected. Responses in CMS and a number of agronomic traits were measured in the F₃ descending families of the selected F₂ plants against the F₃ bulks. Significant responses to selection for CMS were obtained in both directions in the five populations which averaged 19.52% in the high and 11.9 % in the low CMS direction. The realized heritability estimates for CMS which were similar to the calculated parent-offspring regressions ranged from low (0.09 and 0.22) to intermediate (0.34, 0.53 and 0.57). Selection for high CMS produced concurrent positive responses in grain weight per spike in the five populations which averaged 15.72% whereas selection for low CMS reduced grain weight per spike in only two populations with an average reduction of 5.2%. The realized heritability estimates for grain weight per spike ranged from low (0.39) to moderately high (0.70). Significant correlated responses to selection for CMS were obtained in 1000 grain weight in the five populations which averaged 8.25% of population mean in the high direction indicating an improved grain filling capacity. With selection for low CMS, the 1000 grain weight was reduced by 6.09% on average indicating impaired capacity for grain filling. The realized heritability for 1000 grain weight ranged from 0.57 to 0.88. Significant concurrent responses to selection for high CMS were obtained in grain yield per plant only in two of the five populations. Meanwhile, selection for low CMS was not effective in reducing grain yield per plant in four of the five populations. Such limited effect of CMS on grain yield per plant could be attributed to the confounding effect of earliness as an escaping mechanism or heat voidance which might support yield of the unselected base populations as well as the low CMS selections. The realized heritability estimates of grain yield per plant under heat stress were low ranging from 0.09 to 0.43.