Regular gold nanodecahedra (pentagonal bipyramids) are obtained in high yield through ultrasound-induced reduction of HAuCl4 by dimethylformamide on presynthesized seeds (see figure). A dependence of the final morphology on the crystalline structure of the seeds is demonstrated by growing gold on single-crystal platinum nanoparticles, which yields uniform octahedra. Resolution of Maxwell's equations for bicones shows good agreement with experimental optical properties.