Factors accounting for the differential response of five quackgrass biotypes to primisulfuron were compared. Previous field and greenhouse experiments had identified a biotype from Montpelier, OH, as being less susceptible to primisulfuron than biotypes from Amherst, MA; Mahnomen, MN; Fargo, ND; and State College, PA. Differences in shoot, root, and rhizome fresh weights and rhizome bud numbers did not account for the reduced susceptibility of the Montpelier, OH, biotype to primisulfuron. Spray retention was lower in biotypes from OH and MA compared to the others. Absorption of 14C following 14C-primisulfuron application was greater in the Amherst, MA, biotype compared to the others. Only 29% of absorbed 14C was present as primisulfuron in the Montpelier, OH, biotype versus 41 to 45% in the other biotypes 72 h after application. The amount of primisulfuron present in the Montpelier, OH, biotype 72 h after application was less than half the amount present in the other four biotypes due to reduced spray retention and faster metabolism. These factors combined explain lower susceptibility of the Montpelier, OH, biotype to primisulfuron.