A modification of Osterhout''s method of measuring photosynthesis, using a Beckman pH meter and equilibrium equations to determine the CO2 removed from natural water by phytoplankton suspensions, is recommended for studying natural aquatic populations. A study of the communities arising in w. Lake Erie during 1949 (using fluorescent light of 400 f.c.) showed a high correlation between phytoplankton volume and photosynthetic rate, indicating that the major components of these communities were much alike in photosynthetic activity. Respiration rates amounted to 12-14% of total photosynthesis. Total photosynthesis per volumetric unit of phytoplankton was about 1.7 micromoles of CO2 absorbed per 106 standard units per hour (unit is 8 x 103 cu. /j.). It is suggested that the avg. annual standing crop of phytoplankton, when expressed in volumetric units'', may be a fairly reliable index of productivity.