A weekly air sampling program at Fraserdale, Ontario (latitude 50 ¿N, longitude 82 ¿W) has revealed distinct seasonal trends in the isoprene, alkane, and acetylene concentrations over the period April 1990 to October 1992. Concentration of the biogenic hydrocarbon isoprene peaked in July to August <≤5 parts per billion by volume (ppbv)>, with concentrations above 0.1 ppbv from May through September. In contrast, alkane and acetylene concentrations displayed a summer minimum (July to August) and a winter maximum (January to February). Back trajectory data showed that prevailing airflow to Fraserdale was from the northwest sector when samples were collected and throughout most of the year Fraserdale received air originating from northern Canada. An analysis of changes in hydrocarbon concentration ratios over the period provided evidence that the seasonal trend of the C2-C4 hydrocarbons is dominated by seasonal changes in HO reaction rates. In general, the amplitude of the seasonal oscillation for the C2-C5 anthropogenic hydrocarbons increased with HO rate constant. Comparison with Toronto data indicates the long-range transport of acetylene and C2-C5 alkanes of urban origin to this boreal site.