An examination was made of the effects of low-temperature acclimatization on the fatty-acid composition of the polar lipids of two chill-sensitive species Gossypium hirsutum and Phaseolus vulgaris which harden readily against chilling injury. As controls, the effect of hardening temperatures on a chill-resistant species, Hordeum vulgare, and a chill-sensitive species, Episcia reptans, which did not harden were also investigated. The degree of unsaturation of the fatty acids associated with the phospholipid fraction increased during hardening of Gossypium hirsutum and Phaseolus vulgaris. Furthermore, the degree of unsaturation and weight of phospholipids decreased in the leaves of these species with increase in physiological age at 25° C and was related to the increased susceptibility of older leaves to chilling injury. The acclimatization treatment produced no effects on the fatty-acid composition of the glycolipids of these plants and this agreed with earlier observations which showed no increase in unsaturation of the total leaf fatty acid on hardening (Wilson and Crawford, 1974). Therefore, in leaves, it was the phospholipid fraction alone and representing only 22% of the total leaf fatty acids that had its degree of unsaturation positively related to the chilling tolerance of the species investigated. These findings are discussed in relation to similar ones made on the acclimatization of animals to low temperatures.