Evolution of hydrogen ionization fronts expanding into the IGM in a simulation representing the first stage of the reionization history around z = 15. The volume is (5.3 Mpc)³ in comoving coordinates, resolved in 101³ grid cells containing ten randomly distributed clusters of massive population III stars. Each cluster emits 4 × 10⁵³ hydrogen-ionizing photons per second, corresponding to a star formation rate of 9.5 × 10^-3 M_⊙ yr^-1 per comoving Mpc³. The evolution of the ionization fronts starts with a set of discrete H ii regions which begin to overlap after approximately 1 Myr. Hydrogen is completely ionized after 50 Myr (corresponding to Δz ~ 1.5 at z = 15).