Heterotrophic soil microorganisms acquire C and N for maintenance and growth by decomposing plant residuesand other organic materials added to soils. Herbicides with low C:N ratios (, 15) may potentially be readily mineralized, with N that is in excess of microbial demand being released in the inorganic form (Alexander 1977). Glyphosate has a C:N ratio of 3:1 and may have an immediate effect on soil microbial activity. Several previous studies reported that glyphosate applications had no significant effect on soil microbial activity as measured by C mineralization or only had transitory effects when applied at high rates (Hart and Brookes 1996; Olson and Lindwall 1991; Wardle and Parkinson 1990, 1992). Soils in some of these studies, however, were sampled from 1 to 4 wk after glyphosate application and may have missed the primary effect on microbial activity (Hart and Brookes 1996; Olson and Lindwall 1991). Glyphosate persistence in soils has been reported to range from less than 1 mo (Olson and Lindwall 1991) to years (Torstensson 1985). Assessment of C and N mineralization immediately after application may provide a more realistic evaluation of the true effect of this herbicide on soil microbial activity. Nitrogen mineralization from glyphosate has not been reported.