Cell cultures of Nicotiana sylvestris and Capsicum annuum, both without and following exposure to the mutagen, EMS, have been submitted to chilling for 21 days at −3°C and +5°C respectively and the cell lines derived from the surviving cells tested for their subsequent resistance to the chilling treatment. Some of the cell lines when again submitted to the chilling stress showed no enhanced survival, others retained their resistance after an extended period of growth at 24°C. The application of the mutagen promoted the isolation of such stable resistant cell lines. Studies of the response of the respiratory activity of isolated mitochondria to temperature from a resistant and from a sensitive cell line of C. annuum revealed a difference similar to that previously reported from studies on isolated mitochondria from chilling-sensitive and chilling-resistant plants of various species.