Effects of the occupation of higher subbands on the mobility limited by Coulomb scattering and surface-roughness scattering are studied in an n-channel inversion layer on the silicon (100) surface. The first excited subband associated with the two valleys which give the ground subband is assumed to be populated first with increasing electron concentration. Coupled Boltzmann transport equations are solved and intersubband scattering is shown to be very important and to greatly reduce the mobility. Effects of the level broadening are also considered within a simple model of the surface-roughness scattering. A discontinuous drop of the mobility when the excited subband becomes occupied by electrons is almost completely smeared out by the broadening effect. The results explain experimental results of Hartstein et al. quite well, if parameters characterizing the surface roughness are chosen reasonably. © 1979 The American Physical Society.