Epitaxial silicon films have been grown on (100) substrates by the pyrolysis of silane using the molecular beam method. For a given set of growth conditions an induction period, corresponding to removal of the residual surface oxide film as SiO, is required before growth centres form. When subsequently the growth conditions are abruptly changed without switching off the beam, it is found that no further induction period is required (i.e. the surface produced by the first stage is ‘clean') and that a second set of growth centres, commensurate with the new growth conditions, is formed. The conclusions of the previous paper (Joyceet al. 1967) are thus substantiated.