The dominant woody boreal forest plant species were fumigated with 0.34 ppm SO2 under controlled conditions in the laboratory in order to rank their physiological and visual sensitivities to the air pollutant. Deciduous trees and shrubs were much more sensitive than conifers, presumably because SO2 can enter broad leaves much more easily than needles. Labrador tea was intermediate in sensitivity to SO2 and so were its leaf resistances to pollutant uptake. Among conifers, jack pine was more sensitive than either black or white spruce, whereas the species within the deciduous group could not be ranked due to inadequate differences between their tolerance levels.