Field studies were conducted in 1989 and 1990 at St.-Augustin, Quebec, Canada, to determine the economic threshold density of quackgrass in potato. Potato yield losses due to quackgrass interference increased with quackgrass population density. Potato yield ranged from 33 to 73% in 1989, and from 19 to 44% in 1990. The relationship between potato yield losses and quackgrass densities was described by a rectangular hyperbolic function. Dry weight of quackgrass proved to be the best predictor of potato yield loss compared to shoot number. The interference thresholds for 10% potato yield loss amounted to 25 shoots m(-2) or 20 g total dry biomass m(-2). The economic threshold varied between 0.04 and 2 shoots m(-2) or 0.0165 and 1.5 g total dry biomass m(-2), depending on the variables considered