We assessed the presence/absence and population size of a threatened beetle, Osmoderma eremita, inhabiting hollow oaks (Quercus robur). Population sizes varied widely between trees (10% of the hollow trees hosted two thirds of the individuals), and increased with the volume of wood mould (= loose material of dead wood) and the height of the entrances. Population density (number of adult beetles per litre of wood mould) increased with decreasing growth rate of the trees. Trees with the largest O. eremita populations were 300–400 years old. Among hollow trees, the population sizes increased with tree age. This is at least partly due to the fact that the volume of wood mould increased with tree age. Both the size of the largest entrance hole and tree diameter were positively correlated with the estimated wood mould volume, and could thus be used as easily measured proxies for wood mould volume.