The temperature and carrier-density dependent excitonic relaxation in bulk ZnO is studied by means of time-resolved photoluminescence. A rate-equation model is used to analyze the population dynamics and the transitions between different exciton states. Intra-excitonic (n 1) to (n 2) relaxation is clearly identified at low excitation densities and lattice temperatures with a characteristic time constant of 6 ± 0.5 ps.