A 4-year field study was conducted at the Washington State University Turfgrass and Agronomy Research Center at Pullman, WA, to develop a program to remove ‘T-1’ creeping bentgrass from ‘Gallery’ perennial ryegrass with mesotrione (‘Tenacity’4SC).  The perennial ryegrass stand was a green’s surround growing in a Palouse silt loam soil with bentgrass contamination (approx. 70%).  Following 2007 Tenacity applications and overseeding with ‘Auburn Trilogy’, the plots were divided in half; one half left untreated and the other half received multiple Tenacity applications in 2008.  Treatments were not applied in 2009, but were reapplied in 2010.  Treatments were made with a bicycle-wheeled CO₂ pressurized sprayer with 11002 flat fan spray tips at 233 L ha^-1.  Percentages of bentgrass cover and perennial ryegrass phytotoxicity were rated beginning at 1 wk after initial treatment (WAIT) up to 14 WAIT each year.  The experimental design was a randomized-complete block with four replications and experimental units were 1.8 x 2.4 m.  Not all of the creeping bentgrass was eliminated in 2007.  Tenacity applied four times at 140 g ai ha^-1 in 2007 was more effective at reducing the amount of bentgrass than three applications at 140 or 175 g ai ha^-1.  In 2008, 100% bentgrass control was achieved with Tenacity treatments applied four times each year at 140 g ai ha^-1, or 3 times each year at 175 g ai ha^-1.  Bentgrass cover increased in all plots during 2009, prior to reapplication in 2010; however, bentgrass cover was lowest in plots treated four times with 140 g ai ha^-1 in 2007 and 2008.  In 2010, Tenacity applied four times at 140 g ai ha^-1 resulted in complete bentgrass control by mid July.  Phytotoxicity on perennial ryegrass following Tenacity applications was low throughout the study.  Results indicate a multiple year Tenacity program with four applications in each of the first two years at 140 g ai ha^-1 or 3 applications at 175 g ai ha^-1, along with overseeding the first year, is necessary to achieve total bentgrass control.  Subsequent Tenacity applications in following years are needed to prevent creeping bentgrass re-infestation from occurring.