We report on a mode‐locked cw dye laser which reliably produces light pulses as short as 850 fsec. Short‐pulse operation depends critically on the stability and spectral purity of a frequency synthesizer which generates the radio‐frequency signal for the acousto‐optic mode locker of the argon pump laser. The regular axial mode spectrum of the dye laser extends over 500 GHz and can provide an accurate calibration scale for the spectroscopic measurement of large frequency intervals. The high stability of the locked modes has been demonstrated by observing Doppler‐free multiple‐pulse two‐photon spectra of the sodium 3S‐5Stransition linewidths as narrow as 4MHz.