Five perennial cool-season grasses: Kentucky bluegrass, creeping red fescue, tall fescue, orchardgrass, and perennial ryegrass were treated at the 2- to 3-leaf, 3- to 4-leaf, 4- to 6-leaf, and 6- to 7-leaf growth stages with four rates (0, 0.14, 0.28, and 0.56 kg ai/ha) of the butyl ester of fluazifop. Creeping red fescue was not injured by any rate at any growth stage. Reduction in shoot dry weight increased linearly with increasing fluazifop rate for the remaining four species, except for the 6- to 7-leaf stage in Kentucky bluegrass and the 4- to 6-leaf stage in tall fescue and perennial ryegrass. Significant differences between growth stages within each rate varied with species. In general, tolerance increased at the 4- to 6-leaf and 6- to 7-leaf growth stages.