A mutant inbred line of Zea mays L. (M11) in which chlorophyll accumulation is particularly sensitive to low temperature is described. Under natural light conditions the chlorophyll content of seedlings is negligible below 17 degrees but is normal at high temperature. Seedlings of M11 can synthesize chloroplast pigments at 16 degrees but at a rate slower than normal. When photo-oxidation is minimized, chlorophyll accumulates, and seedlings can photosynthesize efficiently at low temperature. The primary site of low temperature sensitivity in M11 is the shoot apex where new leaves are developing and undergoing rapid cell expansion. It seems that there is impaired development and associated impaired function of chloroplasts in M11 grown at low temperatures which sensitizes them to rapid photo-oxidation in the light.