Several seed production fields of the Italian ryegrass cultivar ‘Tetrone’ were destroyed in 1988 by 280 to 350 g ai ha−1 racemic fenoxaprop applied for wild oat control. Because similar rates of fenoxaprop had possessed adequate safety when applied to ‘Oregon common’ Italian ryegrass, suspicion arose that the cultivars differed in tolerance. Seedlings of 21 commonly grown cultivars were screened in the greenhouse at the three-leaf growth stage to determine their fresh weight GR50 for fenoxaprop. The GR50 values for the two most tolerant cultivars, ‘Marshall’ and ‘Torero’, were more than threefold greater than the two most sensitive cultivars, ‘Futaharu’ and ‘Ace’. Cultivars could be separated into sensitive, intermediate, and tolerant groups, but the distribution of the GR50 values appeared to be continuous rather than discrete. Tolerance increased with growth stage, and the average GR50 for tillered plants was 80% higher than that for the two-leaf stage and 41% higher than that for the four-leaf stage seedlings. Cultivars differed slightly in the specific activity of acetyl–coenzyme A carboxylase (ACCase) (EC 6.4.1.2) and in the I50 values for the inhibition by fenoxaprop, but the only clear relationship between these biochemical factors and whole-plant tolerance was a threefold increase in ACCase activity at the tillered stage over that present in the younger seedlings.