Small (20 L) enclosures were used to study the effects of phosphorus addition and zooplankton removal on the phytoplankton of an acidic lake in the Adirondack Mountains of New York. Inspite of inorganic mechanisms which are capable of sequestering phosphate under acidic conditions the phytoplankton population bloomed after the addition of 80 μg L−1of phosphate as KH2PO4. The response was measured by cell counts of the dominant phytoplankter,Chlamydomonasand changes in chlorophyllaconcentration. About half the number of algal cells were present after the two week incubation when zooplankton were not removed, indicating that zooplankton herbivory can influence, but not totally control, the algal production.