A three-year study was conducted to evaluate the effect of defoliation on the yield and quality of sugarbeet grown in south central Montana. Sugarbeet (Beta vulgaris L.) plants were subjected to a single defoliation of 30, 60, or 100% on six dates from July 1 through September 16 in 1991, and seven dates from June 12 and 15 through September 12 and 10 in 1992 and 1993 respectively. Root yield, sucrose content (with the exception of 60% defoliation on July 29, 1993), and sucrose yield (with the exception of 60% defoliation on July 9,1992) were not significantly reduced by 30 or 60% defoliation over the three year study or by 100% defoliation at the mid-June dates in 1992 and 1993. ()ne hundred percent defoliation in late June or early July, mid-July, in mid-August, or in mid-September reduced sugarbeet root yield by an average of 23, 27, 20 and 10%, respectively, averaged over the three-year period. Sucrose content was significantly reduced by 100% defoliation later in the season, from mid-August through mid-September in each year ol the study. Sucrose yield as affected by root yield, sucrose content or both, was significantly reduced by 100% defoliation from late June or July through mid-September in 1991 and 1993, and from late June through August 28 in 1992. Defoliation did not affect sucrose loss to molasses or percent root tare.