Nanocrystalline, single-phase undoped ZnO was sintered to 95%-98% of theoretical density at 650degrees-700degreesC, using pressureless isothermal sintering. The density increased very rapidly at 500degrees-600degreesC, remained constant with sintering temperature until similar to900degreesC, and then decreased slightly. The estimated activation energy for densification at 600degrees-700degreesC (275 kJ/mol) was comparable to grain-growth activation energies previously reported for microcrystalline ZnO but much greater than the grain-growth activation energy measured in the present work. A bimodal microstructure, consisting of nanocrystalline grains within larger ensembles ("supergrains"), was observed, and both modes grew as the sintering temperature increased. The grain-growth activation energy for the nanocrystalline grains was extremely low, similar to20 kJ/mol. The activation energy for the growth of the supergrains depended strongly on temperature but was similar to54 kJ/mol at >500degreesC. The important mechanisms probably are rearrangement of the nanoparticle grains, with simultaneous surface and boundary diffusion, and vapor transport above 900degreesC.