The specification of moisture contents in forest fuels is an integral part of any workable fire-danger rating system. This paper presents a linear model for estimating the moisture content of the 100-hour timelag fuels. The variables in the model include yesterday's computed value for 100-hour timelag fuel moisture, today's observed 10-hour timelag fuel moisture and a binary variable which is set if rain has occurred in the 24 hours prior to the observation time. The standard error of estimate for a data set of 69 points including 25 days during which some precipitation occurred was 0.65 percent.© Society of American Foresters, Bethesda, MD. Abstract reproduced by permission.