Colonization of a relatively undisturbed 1915 debris flow by an upper montane coniferous forest was examined in Lassen Volcanic National Park in northern California in 1987. Seventy 100-m² circular plots were arranged in four transects across the flow and sampled to measure tree densities, heights, basal areas and ages. The composition of the forest changed from a mixture of Abies magnifica, Pinus monticola and P. contorta on steep slopes to a forest dominated by P. contorta on shallow slopes. This pattern is typical for these terrains at the 2000-m elevations of the flow. Age data and historical photographs indicated little successful colonization before the late 1930s and peak colonization rates about 1955. Height growth has generally been slow with most trees being >20-y-old but <2-m tall; however, some individuals, including some recent colonizers, have shown rapid growth. This variation among individuals suggests (1) that the earliest colonizers are not necessarily those which will eventually dominate the forest and (2) that opportunities to invade and occupy the canopy may extend for 30 y after the first successful colonization.