Cubic ZnSe nanowires with periodically alternating twins along the wire growth direction (see Figure) have been synthesized using thermal evaporation with a Au catalyst. The periodicity of the alternating twins has a linear dependence on the diameter of the nanowires. Sharp excitonic peaks dominating the low-temperature photoluminescence spectrum of the nanowires reveal the high quality of their electronic structure despite the large unpassivated surface and interface associated with the twinned nanowire configuration.