Silicon nanowhiskers in the diameter range of 70 to 200 nm were grown on <111>-oriented silicon substrates by molecular-beam epitaxy. Assuming the so-called "vapor-liquid-solid" (VLS) growth process to operate, we initiated the growth by using small clusters of gold at the silicon interface as seeds. The in situ generation of the Au clusters as well as the growth parameters of the whiskers are discussed. The experimentally observed radius dependence of the growth velocity of the nanowhiskers is opposite to what is known for VLS growth based on chemical vapor deposition and can be explained by an ad-atom diffusion on the surface of the whiskers. ?2004 American Institute of Physics.