The absorption coefficient for polarized light at photon energies less than that of the lowest lying direct exciton was measured for single crystals of ZnO at temperatures ranging from 20° to 200°K. Analysis of the results shows that the absorption is in agreement with that calculated for a process involving the simultaneous creation of an exciton and absorption of a phonon, both particles having a small wave vector. This agreement provides evidence that the absolute minimum of the conduction band cannot lie lower than the lowest lying direct exciton level, and, therefore, that the absolute minimum probably occurs at the center of the Brillouin zone. Values for the hole and electron masses were estimated from the analysis.