In order to perform ultrahigh-resolution spectroscopy, a tunable laser with a narrow spectral width is required.In this paper we describe a single-frequency jet stream cw dye laser that is carefully designed to min-imize high-frequency laser jitter. The re-sidual low-frequency jitter is reduced to 200 kHz rms by locking the laser frequency to an external reference cavity. The stability of the laser has been verified by observing extremely narrow resonances (700 kHz FWHM) in an iodine molecular beam. The laser frequency can be linearly and smoothly tuned by varying the length of the external cavity. In certain precision spectroscopic applications long-term laser stabilization is needed for the generation of secondary wavelength standards throughout the visible region. By locking the dye laser to a hyperfine transition in a molecular beam of I2 we have demonstrated a long-term stabilization of 6 parts in 1013.