The herbicide glyphosate (MON-0139), an isopropylamine salt of N- (phosphonemethyl)glycine, was tested to determine its effects on the growth of the fresh water green alga Selenastrum capricornutum. Experiments were carried out to determine algicidal, algistatic, and inhibitory concentrations of the compound. Concentrations from 5 ppm to 1000 ppm were inhibitory to the growth of the alga, but the effects were algistatic, rather than algicidal. At concentrations from 0.1 ppm to 1.0 ppm the growth of the alga was stimulated by the presence of the herbicide, while at concentrations from 0.001 ppm to 0.05 ppm the effects of the herbicide were negligible. Two chemical tests were used, and it was determined that in low pH systems (pH of about 4.5) glyphosate hydrolyzes with inorganic phosphate as one of the hydrolysis products. The concentrations of phosphate released is so low as to have no effect on the growth of S. capricornutum.