The paper presents the results of a gas station survey in the Spring of 1978 which attempted to estimate the incidence of the misfueling of vehicles. Misfueling was defined as the use of leaded gasoline in cars designed to use unleaded gasoline. The data was collected by on-site observers in seven major Canadian cities. In addition to fuel type used, information on the frequency of pump nozzle substitution and fuel inlet tampering are compiled and presented. Conclusions on the importance of this problem in the control of emissions from cars are developed.