An empirical representation of the power spectral density of horizontal gustiness near the ground in high winds is combined with a theoretical model for the response of free-burning fires to nonsteady wind to predict the variability of spread rate and intensity of wind-aided free-burning line fires in natural fuels. A nearly periodic variation of intensity at very low frequency (fixed by fuel description) is suggested by these results. Spread rate variability is found to be rather more erratic, with standard deviation exceeding the mean value for short periods of time in many fuels. Field experience suggests that this prediction is realistic. These results have significant implications for wildland fire research and safety planning for fire suppression. ('This manuscript was written and prepared by an employee of the U.S. Government on official time and is therefore in the public domain.')