Satisfactory selectivity of 4-chloro-2-butynyl m-chlorocarbanilate (barban) for the control of wild oats (Avena fatua L.) in spring wheat was obtained when it was applied from 4 to 14 days after emergence of wild oats. Following this period, the effectiveness and selectivity declined rapidly. Depending on the season, this period of maximum efficiency represented the following growth stages of wild oats; 1 to 3-leaf stage in 1960, 1 1/2 to 3 1/2-leaf stage in 1961, and 1 1/2 to 2 1/2-leaf stage in 1962. Injury to wheat was most pronounced during a period beginning about 15 days after crop emergence and continuing for several weeks. This pattern of wild oat control and wheat susceptibility was virtually identical over the 3-year period even though growing conditions varied considerably. This suggests that days after emergence may be at least as reliable as leaf stages as an indexfor the proper timing of barban treatments and would be more easily determined under practical field conditions. If leal stages are used, then spraying should not be recommended later than 2 1/2 -leaf stage, since in some years, wild oats will have reached resistant stages. Furthermore, it may be desirable to apply barban early, perhaps even before all the plants are fully in the 2 1/2-leaf stage. When wild oats reach the 2-leaf stage, it may be too late, at least in years of abundant moisture.