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iv 0.1% deicer. Both mortality and reproductive tests were repeated with 1997-98 deicer; results were similar to those of the previous year. 12. The algal genus Selenastrum was tested with 1997-98 deicer for response to magnesium chloride deicer over intervals of 96 hours. The test showed significant suppression of division rate for the algal cells occurring at deicer concentrations slightly in excess of 0.1%. Other indicators of physiological stress appeared at concentrations of approximately 1%. 13. In overview, toxicity tests show that various kinds of aquatic organisms differ in their sensitivity to magnesium chloride deicer. The most sensitive kinds of organisms included in these tests begin to show observable effects at about 0.1% magnesium chloride deicer during exposures ranging from 48 to 168 hours. Because of the presence of melt water, magnesium chloride deicer applied to roadways is diluted to approximately 0.2% prior to leaving the roadway, and an additional amount (to less than 0.1%) within short distances (e.g., 20 yards) of the roadway. 14. Mass transport was estimated from field data for magnesium, chloride, and sodium at 6 field sites (stream study segments) during 1997 and 1998. The amount of magnesium added in the form of magnesium chloride to roadways greatly increased the total annual transport of magnesium. Addition of magnesium chloride raised the concentrations of magnesium in streams by as much as three times above baseline concentrations of 2-3mg/L. Winter concentrations were most strongly affected bemuse stream discharge is low during winter, and thus dilutes the magnesium less than during spring. Even though changes in concentration were substantial, they fell well within the natural range of magnesium concentrations in Colorado waters and raise no specific environmental concerns.