Algal blooms in our streams, lakes and reservoirs have been known to cause nuisance problems. Water treatment problems include clogging of filtration units, taste and odor, turbidity and green coloration of water. Dense algal blooms reduce the recreational value of water and, recently, have been reported to kill fish. Algal blooms have been attributed to increased nutrients in waterways from wastewater discharges and runoff from agricultural land which contains fertilizers and herbicides. A widely used herbicide, 2,4 dichlorophenoxyacetic acid (2,4D), has been found in waters adjacent to land applications in concentrations known to be stimulatory to many plants. The purpose of this study was to evaluate the effect of 2,4D on the growth of Chlorella pyrenoidosa in batch cultures. Cultures were grown over a 12 day period in 2,4D concentrations of 10⁻⁸ to 2 x 10⁻³ M. The culture medium was inoculated to provide 2.0 x 10⁴ cells/ml and incubated under continuous fluorescent lighting at 140, 200 and 220 foot candles and 20 ± 1.0°C. Cell counts and fluorometric readings were made on even days and analyzed by least squares regression and analysis of variance. The results indicate that 2,4D promoted growth of Chlorella pyrenoidosa. Increased growth was discovered with increases in 2,4D concentration from 10⁻⁸ to 2 x 10⁻³ M 2,4D. A concentration of 2 x 10⁻³ M 2,4D appeared to cause slight inhibition when compared to 10⁻³ M 2,4D. Light intensity had little effect on 2,4D utilization by Chlorella. The results of this study indicate that herbicides used currently for weed control may be correlated to increase algal growth and thereby influence water quality. / Master of Science