Mesotrione [2-(4-mesyl-2-nitrobenzoyl)-3-hydroxycyclohex-2-enone] is relatively a new pigment inhibitor herbicide that is labeled for use in corn (Zea mays L.). This herbicide controls susceptible weed species by inhibiting the 4-HPPD enzyme. Mesotrione is effective in controlling many of the broadleaf weeds found in turfgrass environments. It is expected to be labeled for weed control in turfgrass. Limited information is available on the tolerance of Kentucky bluegrass cultivars. Our objective was to identify the response of various Kentucky bluegrass cultivars to mesotrione.The National Turfgrass Evaluation Program at the University of Massachusetts in South Deerfield consisted of 172 cultivars of Kentucky bluegrass. These plots were established in 2000, and maintained with fertility and other management practices. In 2006, a field trial was conducted with these 172 cultivars. Plots were 3 by 6 feet in size. This trial was a split-plot design with four replications having cultivars as the main plot and two mowing heights (0.5 and 1.5 inch) as sub-plot. Experimental area was mown twice a week and the clippings were removed. Mesotrione was applied POST to Kentucky bluegrass at the rate of 840 g ai/ha on July 18, 2006. Treatments were applied using a CO2-backpack sprayer with Teejet XR 1104 VS nozzles at 152 kPa using a spray volume of 467 L/ha. Turfgrass injury was visually estimated on a scale of 1 to 9, 1 = dead turfgrass, while 9 = healthy green turfgrass. Injury ratings were made 1, 2, 3, and 4 weeks after treatment (WAT).In general, Kentucky bluegrass exhibited a wide range of tolerance to mesotrione.