Creeping bentgrass creates a dense, high-quality playing surface on golf courses, but it often encroaches adjacent areas of Kentucky bluegrass. Mesotrione [2-(4-mesyl-2-nitrobenzoyl)-3-hydroxycyclohex-2-enone] can kill creeping bentgrass in Kentucky bluegrass, but more information is needed regarding the application protocol. We conducted research at the Iowa State University Horticulture Research Station near Gilbert, IA. Our objective was to determine the effect of mesotrione rate and number of applications on creeping bentgrass kill in Kentucky bluegrass and on overall turfgrass quality. Mixed swards of creeping bentgrass and Kentucky bluegrass received one or two applications of mesotrione at 0, 70, 140, 280, 560, 840, and 1120 g•ha^-1. One application of mesotrione killed 32% to 88% of creeping bentgrass in Kentucky bluegrass. Creeping bentgrass kill increased to 75% with mesotrione applied at 560 g•ha^-1. Two applications of mesotrione at 70 to 1120 g•ha^-1 each, killed 38% to 99% of creeping bentgrass in Kentucky bluegrass. Creeping bentgrass kill increased with increasing application rates up to 280 g•ha^-1 each, which provided 86% kill. One application at 560 g•ha^-1 or two applications at 280 g•ha^-1 each, reduced overall turfgrass quality 44% for only one week. Two applications of mesotrione at 280 g•ha^-1 provided effective postemergence control of creeping bentgrass in swards of Kentucky bluegrass.