In this paper we present a fabrication route to produce size-selected III-V semiconductor nanocrystals via a simple, reliable, and efficient aerosol route. Since this approach includes the reaction of aerosol particles and a self-organized growth of a new compound, all in the aerosol phase, we call this process aerotaxy. Size-selected nanocrystals of different m-V compounds in a diameter range below 20 nm were fabricated using this method. Through the reaction of arsine with gallium droplets or of phosphine with indium droplets, GaAs and InP clusters were formed. Our approach opens the possibility to produce contamination-free and size-selected nanocrystals of compound semiconductor materials with considerable freedom in composition and size.