Tolerance of barley to chlorsulfuron at various stages of crop development was studied in weed-free field and pot experiments. Five barley crops were sown between mid-May and mid-July in the field in 1988 and sprayed simultaneously with four rates of chlorsulfuron when the corresponding stages of crop development ranged from growth stage 7 (GS 7) to GS 31. Early-sown barley yielded more than late-sown barley, but percent grain yield reduction from chlorsulfuron was similar for all planting dates. In a field experiment in 1989, barley was sown at three dates (May, June, and July) and each was treated with chlorsulfuron at 0, 15, 30, and 60 g ai ha−1 at either GS 12 or GS 15. Grain yield of barley sown in May and treated with 15 g ha−1 chlorsulfuron at GS 12 was reduced 35%, compared with no effect when applied at GS 15; this yield reduction was related to spike size and number. Application of chlorsulfuron at GS 12 and GS 15 to barley sown in June caused yield reductions of 20 and 11%, respectively. In contrast, barley sown in mid-July was not affected by chlorsulfuron. Applications of chlorsulfuron at GS 12,13,14, and 15 led to reductions of 75,35,15, and 5% shoot dry matter (DM) of plants grown in pots and measured at GS 33. Root DM was more sensitive to chlorsulfuron than shoot DM. Tolerance of barley to chlorsulfuron increased with age of the plant at the time of application.