Semiconductor nanowires (NWs) growth is typically assisted by a metal particle, called the catalyst. The use as the catalyst of a material different from the components of the wire may change the semiconductor properties due to the diffusion of the catalyst in the nanowire body during the growth. Moreover, the most commonly used catalyst is Au, a metal that is incompatible with the Si technology. For the above mentioned reasons it is therefore of importance to develop a technology leading to a catalyst-free growth of semiconductor nanowires. Here we report preliminary results on catalyst-free growth of GaAs NWs by molecular beam epitaxy. The GaAs NWs have been grown on cleaved edges of Si wafers, with no catalyst pre-deposition. The growth lasted 30 minutes and has been performed at 580 T 620 °C. Two kinds of nanowires have been obtained. The NWs of the first type are as long as 5 μm with a section diameter in the range of tens of nm and have a spherical particle at their end tip. Energy dispersive X-ray spectroscopy (EDS) demonstrates that the spherical particle is composed of Ga, and that the NW body is GaAs. The NWs density depends on the crystallographic orientation of the facets that compose the cleaved edges of the Si-wafers. The second type of NWs are generally characterised by a smaller aspect ratio, clearly faceted lateral and tip surfaces, and no metallic particle on their tip. EDS curves reveal that they are completely made of GaAs. The EDS results suggest that a Ga induced self-catalysed growth occurred on specific surface locations where Ga droplets formed and were trapped during the first stages of the GaAs deposition. Work is in progress to understand the growth process and in particular to understand whether the droplet-less NWs grow through a different process or the absence of a Ga droplet is due to its lost during growth.