Relationships among the phytoplankton standing crop, rate of photosynthesis, and the distribution of C—14 in primary products of photosynthesis were investigated in western Lake Erie from July to August 1965. Sixty species of phytoplankton were noted during the study, but only four species, Coscinodiscus sp, Ceratium hirundinella (O.F.M.) Schrank, Aphanizomenon flos—aquae (L) Ralfs, and Microcystis aeruginosa Kutz accounted for over 98% of the total cell volume. Cell volumes ranged from 10 to 282 mm3 liter—1. Total photosynthesis (mgC m—3 day—1) in the upper photic zone averaged 566, ranging from 55 to 1690. Relative photosynthesis (mgC day—1 mm—3) ranged from 1.6 to 332. Ethanol—soluble C—14 fractions (mostly carbohydrate and lipids) of the phytoplankton accounted for 25—60% of the total C—14 assimilated. Insoluble C—14 fractions (mostly proteins) accounted for 20—48% of the total C—14 uptake. Large carbohydrate and small protein fractions occurred in Coscinodiscus standing crops under moderate illumination and in steady—state Alphanizomenon—Microcystis standing crops under high illumination. C—14 labeled protein fractions exceeded carbohydrate fractions in rapidly growing Aphanizomenon—Microcystis standing crops under moderate illumination and at the peak of a blue—green algae bloom under low illumination.