At present the only rapid chemical method known for estimating living plant matter in the particulate organic matter of sea water is to determine is to determine the characteristic plant pigments-the chlorophylls, carotenes and xanthophylls. Unfortunately the amount of organic substance associated with a given quantity of plant pigment is very variable, depending upon the class of the phytoplankter and its state of nutrition (the factor for converting chlorophyll a to total plant carbon can vary between about 25 and 100.) the method described below determines the three chlorophylls commonly found in planktonic algae, chlorophylls a, b, and c. the carotenoid pigments (the carotenes and xanthophylls) are only estimated collectively in somewhat arbiter units. If the plant population contains many myxophyceae some foems of phycobilin pigments may extract and interfere with all determinations except that of chlorophyll a. Fortunately this occurs only rarely in truly marine waters.