A semimicro spectrophotometric method is described for the estimation of chlorophyll a, b, and c and of astacin and nonastacin type carotenoids in acetone extracts of plant and animal material. Developed specifically for use in estimating and characterizing plankton populations, the method is highly sensitive and practical for shipboard use. Methods of collecting, preparing and extracting plankton samples, spectrophotometric measurements, computation of results, and errors are discussed.