A detailed set of data has been compiled on large fire whirlwinds occurring on prescribed burns conducted in Ontario. There appear to be two types of such whirlwinds: one occurs in pairs on the leeward side of the convection column and the other is created after the entire convection column begins to rotate. The second type occurs in association with very intense fires that may be described as fire storms. Fire whirlwind occurrence appears to be related principally to meteorological conditions in which wind speeds are less than 10 km/h, to the stability of the atmosphere up to 3000 m altitude, and to conditions where the amount of energy released from the fire is high. The roles of atmospheric stability, rate of energy release from the fire, and ignition pattern in the development of whirlwinds require further study.