Seedlings (var. Badger Market) were grown in growth chambers (25[degree] day, 15[degree] night) and hardened for 6 weeks at successively lower temperatures from +5 to -3[degree]. The maximum hardiness attained was a frost killing point of -20[degree], as opposed to a maximum of -7 to -10[degree] for greenhouse-grown plants hardened at +3[degree]. The effects of photoperiod on growth of the cabbage plants were apparently chiefly if not solely due to their effect on total net photosynthesis accumulated per day. Frost hardiness was related directly to photoperiod (8, 12, 18, 24 hours) up to the end of the 1st week of hardening, but inversely from the end of the 2nd week and for the next 2-4 weeks. Growth characteristics such as dry matter and morphological characters usually correlated with hardiness were correlated only up to the end of the 1st week of hardening. Osmotic potential was correlated with hardiness at all photoperiods up to the end of the 3-6 weeks of hardening at +5 to 0[degree], during which lights were used, but not during the succeeding 3-6 weeks of hardening at -3[degree] in the dark.