A simple probabilistic model for the destruction of forest stands by fire is used to determine a likelihood function for the problem of estimating the per annum probability of stand-replacement fire using data in the form of areas in various age classes of undisturbed old-growth forest. This enables the estimation of either an age-independent fire probability or of the parameters of a parametrically defined age-dependent probability. The model includes contagion effects (the probability of a stand burning being increased by other stands burning). Maximum likelihood is used to obtain estimates of the age-independent per annum probability of stand-replacement fire (or the parameters of an age-dependent model), along with a contagion parameter. Confidence intervals for the fire probability are also obtained.