ZnO is commonly observed and assumed to crystallize fully in a wurtzite structure. Indeed, While polytypism between a wurtzite and a zinc blende phase is often seen in some II-VI compounds, it has only rarely been observed in ZnO. Here, by means of automated crystal orientation mapping and high resolution imaging in transmission electron microscopy, we identify nanometer-sized regions of zinc blende forming during the growth of compact nanocrystalline metal organic chemical vapor deposited ZnO films. We analyze a c-fiber-textured wurtzite ZnO film, where the columnar grain growth, typical for polycrystalline films, is interrupted by the formation of these zinc blende regions. These regions in turn provide surfaces for the heteroepitaxial renucleation of newly oriented wurtzite grains. In contrast to the columnar grains, which grow fastest along their c-axis, these new grains are identified to grow fastest along their basal plane. By means of convergent beam electron diffraction, we show that this difference in fast growth direction relates to the polarity of exposed facets. A growth model is proposed, showing how this polytypism affects the microstructure evolution of the film in ways that were not predicted by existing film growth models. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.