Communication: Semiconductor clusters, particularly gallium arsenide, have potential applications in nonlinear optical devices. Special interest is generated by the fact that their optical and electronic behavior depends on the size of the cluster. Direct evidence is presented for the occurrence of hexagonal, i.e., wurtzite-type structure, GaAs in clusters, whereas bulk GaAs has zincblende structure. High-resolution transmission electron microscopy, atomic force microscopy and electron diffraction results are reported and interpreted. Suggestions are made for further work.