During 1949–57, surveys and resurveys were conducted throughout one-third of the settled area of Saskatchewan for the purpose of ascertaining the abundance of several species of introduced persistent perennial weeds and to evaluate their rates of increase. The methods used included an extensive farm-to-farm survey, sampling quarter-sections along transects in heavily infested areas, and resurveys of the same land at intervals. Detailed studies along line transects in various habitats were used during 1954–61 to obtain more precise data concerning increases and decreases in extent of infestations. These studies indicated that, within the area surveyed, 142,702 acres were infested with toadflax (Linaria vulgaris Mill.) in 4,993 quarter-sections. The increase in abundance of the weed was particularly great from 1949 to 1954, while after that date uncultivated infestations spread much less rapidly and cultivated ones were kept under fairly good control. The data indicate that the weed probably responds to weather conditions in such a way as to be favored relative to grasses by deficiency of soil moisture in the upper levels of the soil.