Isolated cobalt nanoparticles have been obtained by annealing Co films produced by the electron-beam evaporation of a solid target onto a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. Preliminary ion bombardment of the substrate reduces the average size of the particles formed during coalescence, but the shape of large particles is distorted. It is established that the Co nanoparticles retain their shape when exposed to air. Magnetic force images show that the particles of 20-40 nm in height are single-domain.