Greenhouse and laboratory studies were conducted to determine the degree of dominance of the monogenic sulfonylurea herbicide resistance trait in diploid sugarbeet by comparing the response of homozygous and heterozygous resistant sugarbeet to primisulfuron, thifensulfuron, and chlorimuron on the whole plant and acetolactate synthase (ALS) enzyme level. Progeny tests suggested that the monogenic sulfonylurea herbicide resistance was semidominant. Subsequently, heterozygous resistant (R-1) and homozygous resistant (R-2) sugarbeet lines were sprayed with increasing rates of primisulfuron, thifensulfuron, and chlorimuron, and herbicide rates required for 50% growth reduction (GR50) were determined. GR50 values were also determined for homozygous susceptible sugarbeet lines (S-1 and S-2). The GR50 values indicated that the R-2 sugarbeet was 377, 269, and 144 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than susceptible S-2 sugarbeet. In contrast, R-1 sugarbeet was only 107, 76, and 57 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than S-1 sugarbeet, indicating at least a twofold difference in the magnitude of resistance between homozygous resistant and heterozygous resistant sugarbeet lines. ALS enzyme activity analysis were consistent with whole plant results. Thus, based on these two, maximum crop resistance can be obtained by developing homozygous resistant cultivars.