A method is needed by which biomass production resulting from non-photosynthetic micro-organisms in natural aquatic ecosystems can be included in microbial production measurements. Both laboratory cultures and field samples were used for examining the suitability of measuring sulfate uptake as a yardstick of microbial production. Sulfate uptake, using 35SO4, was com pared with 14CO2 uptake and it was found that sulfate uptake measures significantly more microbial production than does 14CO2 uptake. With further study, the use of sulfate uptake may become a very satisfactory method for better estimating the total planktonic microbial production under aerobic conditions.