Mode-locking in a cw narrowband tunable dye laser whose cavity contains absorbing cells with molecular iodine vapor has been studied experimentally. When the wavelength of radiation of the dye laser is tuned to absorption lines of molecular iodine vapor in a cell in the cavity, short pulses following with a period equal to or multiple of the round-trip time of light around the cavity appear. The duration of the generated pulses is about 1 ns, which is much shorter than the phase memory time in iodine vapor. In such a situation, mode- locking occurs owing to the coherent character of the interaction of laser radiation with narrow absorption resonances in molecular iodine vapor.