The controllable synthesis of semiconductor nanocrystals is important for exploiting their size-dependent properties in a variety of applications. The important wide-bandgap semiconductors ZnS and ZnSe crystallize in both the zincblende (ZB) and wurtzite (WZ) structures. While the ZB polymorphs are most stable, methods exist for synthesizing the WZ-type nanocrystals. However, because of the subtle structural differences between the ZB and WZ structures, subtle synthetic differences can favor one polymorph over the other. It is therefore challenging to predictably generate the WZ polymorphs and understand the factors that play a key role in their formation. Through careful mechanistic studies, we show that ZnO nanoparticles, which adopt the WZ structure, form as intermediates in typical reactions that generate WZ-ZnS. This implies that ZnO nanoparticles can serve as structural templates for the preferential formation of WZ-ZnS nanoparticles, and this is confirmed experimentally. Similar chemistry can be used to preferentially form WZ-ZnSe and ZB-ZnSe.