Rich populations of clusters have been observed after fermosecond laser ablation of bulk silicon in vacuum. Size and velocity distributions of the clusters as well as their charge states have been analyzed by reflectron time-of-flight mass spectrometry. An efficient emission of both neutral silicon clusters Si, (up to n = 6) and their cations Si-n(+) (up to n = 10) has been observed. The clusters are formed even at very low laser fluences, below the ablation threshold, and their relative yield increases with fluence. We show the dependencies of the cluster yield as well as the expansion dynamics on both laser wavelength and laser fluence. The mechanisms of the cluster formation are discussed. (C) 2003 Elsevier B.V. All rights reserved.