In aerobic waters, the uptake of sulfur by microorganisms should, theoretically, reflect their uptake of carbon. Studies with pure cultures of bacteria suggest that sulfate sulfur uptake follows the kinetics of organic carbon uptake and, also, that sulfate sulfur is the only significant source of sulfur in natural microbial biomass production. Sulfate uptake should, therefore, be useful for measuring microbial heterotrophy. However, partitioning photosynthetic from heterotrophic sulfate uptake is complicated, since dark sulfate uptake cannot necessarily be interpreted as heterotrophy. Ongoing studies with axenic cultures suggest that a relationship between primary production and sulfate uptake exists, thereby potentially allowing the calculation of heterotrophy in terms of the construction of sulfur biomass. Such calculations, although preliminary, are presented.