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Microcystis aeruginosa is known widely for the obnoxious nature of its bloom. Problems resulting from blooms of the alga include: death of fish and other aquatic life, clogging of filter systems in water treatment plants, taste and odor problems, and death of cattle and water fowl via extracellular polypeptides. Blooms of this nuisance alga have historically occurred in waters receiving pollution from sewage effluents or runoff from agricultural lands. Studies have shown that 2,4-0ichlorophenoxyacetic acid (2,4-0) and 2,4,5-Trichlorophenoxyacetic acid (2,4,S-T) mimic the metabolic activities of the plant hormone indoleacetic Acid (IAA). IAA has been shown to stimulate the growth of some green and blue-green algae. Recent evidence suggests that blooms of Microcystis aeruginosa were triggered when the herbicides 2,4-D and 2,4,5-T were introduced to lakes and rivers. The results of this study showed that batch cultures of non-axenic Microcystis aeruginosa containing 2,4-D and 2,4,5-T exhibited significantly more growth than controls over a wide range of herbicide concentrations. Growth in 2,4-D treated cells was statistically significant from the control at a = 0.05 at concentrations as low as 10 -7 M (0.020 mg/l) and 10 -5M (2.5 mg/l) for 2,4,5-T. Also, a plot of maximum cell yield values versus herbicide concentrations resulted in a linear relationship. / Master of Science