Trifolium subterraneum (cv. Dinninup) responds to enriched atmospheric CO2 in a manner similar to that described by Madsen (1968 and 1976) for tomato. In immature leaves, the total chlorophyll content per unit dry weight and the chlorophyll a:b ratio are significantly lower in plants grown at 0.10 vol% CO2. Although fully expanded mature leaves partially overcome the deficit in chlorophyll content, the chlorophyll a:b ratio remains substantially lower in these high CO2 grown plants. The large amount of starch accumulated as irregularly shaped grains appears to disrupt normal chloroplast structure in clover plants grown in enriched atmospheric CO2. These results indicate the chlorotic appearance of leaves from high CO2 grown clover plants is due to a decrease in chlorophyll content per dry weight possibly resulting from large starch grains and starch accumulation altering normal chloroplast structure and function.