The presence of up to 500 μg sulfur·l-1 of an equimolar mixture of cysteine and methionine had virtually no effect on the SO42- uptake rate of Navicula pelliculosa, (Br\ eb.) Hilse whereas the rate of Ankistrodesmus falcatus (Corda) Ralfs was decreased by the presence of 500 μg S· l-1 and Anabaena flos-aquae (Lyngbye) Br\ eb. by 50 μg S·l-1. Primary productivity in these axenic cultures was affected (decreased) only in A. falcatus. The C:S uptake ratio was lowest in N. pelliculosa and highest in A. falcatus. Considering these species as representative of groups of naturally occurring algae, patterns of SO42- uptake and primary productivity in a eutrophic and a moderately oligotrophic lake reflected the results of the algal culturing experiments: SO42- uptake rates, relative to primary productivity, were higher in the presence of diatoms and bluegreen algae and lower when green algae were present; the addition of the cysteine I methionine mixture to the lake waters decreased the rate of microplankton SO42- uptake in correlation with the makeup of the algal community; primary productivity decreased upon the addition of cysteine I methionine when green algae were relatively abundant. It is concluded that, in most fresh water systems, the effects of organic sulfur pollution on algal SO42- uptake and primary productivity are insignificant as compared to other ecological changes that occur due to that pollution.