Representative plants containing either the reductive pentose phosphate cycle or the C(4) dicarboxylic acid cycle of photosynthetic carbon dioxide fixation have distinctly different contents of P(700) and chlorophylls a and b. With leaf extracts and isolated chloroplasts from C(4) cycle plants, the mean value of the relative ratio of P(700) to total chlorophyll was 1.83 and the mean value of the ratio of chlorophyll a to b was 3.89. The respective values in similar extracts and chloroplasts from pentose cycle plants are 1.2 and 2.78.It seems likely that these results are indicative of a more active Photosystem I or a different size photosynthetic unit in C(4) cycle plants than in the reductve pentose phosphate cycle plants.