The occurrence and behavior of lightning-caused fires in Yellowstone National Park wer summarized for 17 years (1972-1988) during a prescribed natural fire program. Both ignition (occurrence) and spread (Stand replacing fire activity) of fires were strongly influenced by fuel mositure and forest covery type. Fuel moisture estimates of 13% for large (>7.6 cm) dead and downed fuels indicated a threshold below which proportionately more fire starts and increased stand replacing fire activity were observed. During periods of suitable fuel moisture conditions, fire occurrence and activity were significantly greater than expected in old-growth, mixed-canopy lodgepole pine and Engelmann spruce - subalpine fir forest types, and significanlty less than expected in the successional lodgepole pine forest types. During periods of extended low fuel moisture conditions (drought), substained high winds significantly reduced the influence of forest cover type on stand replacing fire activity. These extreme weather conditions were observed during the later stages of the 1988 fire season, and to a lesser extent, for a short duration during the 1981 fire season. The Douglas-fir forest type typically supported little stand repalcing fire activity, even though a preponderance of fire starts was observed.