Historical data for NW Ontario were used to determine relations between catastrophic (stand replacing) fires and landscape diversity in the boreal forest. Results showed that landscape diversity was greatest with intermediate amounts of fire disturbance. Any change of disturbance rate will reduce landscape diversity. The data showed gross variation in forest fires over time, with clusters of severe fire years and of quiescent years. It is suggested that an increase in numbers of fires will occur in response to climate warming. Fighting such fires in timber production areas is amply justified; in areas not prone to fires and of marginal forestry potential such fires should be allowed to burn to increase habitat diversity without harming forestry interests.