Cordgrasses (Spartina sp.) are exotic, invasive species that threaten to degrade the intertidal zones of estuaries along the West Coast of North America. Integrated pest management (IPM) strategies primarily focus on the use of aerial and ground applications of Rodeo in conjunction with mowing, but IPM treatments over multiple years usually are necessary to control Spartina. Although information exists regarding the short-term fate and effects to marine biota of a single Rodeo application to control Spartina, little information is available regarding the fate and biotic effects associated with repeated Rodeo applications necessary for control. Consequently, we conducted a 3-year study to assess the short- and long-term fate and potential effects to marine biota associated with repeated applications of Rodeo to control smooth cordgrass in a southwestern Washington estuary. At each of three intertidal locations in Willapa Bay, we established plots on exposed mudflats and along the edge of a Spartina meadow that were hand sprayed with Rodeo (5% solution) and LI-700 (2% solution) during July 1997 and 1998. Glyphosate concentrations in sediment from mudflat plots declined 88% to 96% from 1 day posttreatment in 1997 to 1 year after the second Rodeo applications in 1999. In contrast, glyphosate concentrations in Spartina plots increased 231% to 591% from 1997 to 1999 because Spartina rhizomes likely did not readily metabolize or exude it. Comparison of concentrations from mudflat and Spartina plots with toxicity test values for marine biota indicates that under worst-case conditions short- and long-term detrimental effects to aquatic biota from repeated application of Rodeo for Spartina control would be highly unlikely.