The seasonal variation of the different fractions of photosynthesizing phytoplankton, measured by C-14 uptake, was studied in a mangrove swamp region, at 25º south latitude. The data showed that the fraction with size range from 5 µ, to 65 µ, was responsible for an average of 61.80% of C-14 uptake of the phytoplankton, during one year, samples having been taken every other month, and showed little seasonal variation when compared with the fraction larger than 65 µ. The results presented here, confirmed general conclusions of other authors, and provided a back-ground for future investigation in the area.