Experimental observations of the lasing spectrum of a single mode semiconductor laser under continuous microwave modulation reveal that the lasing spectrum is apparently locked to a single longitudinal mode for optical modulation depths up to ∼80%, beyond which the lasing spectrum becomes multimoded, whose envelope width increases very rapidly with further increase in modulation depth. These results are satisfactorily explained by a theoretical treatment which enables one to predict the dynamic lasing spectrum of a laser from its cw lasing spectra at various output powers.