An actively mode-locked fiber laser is demonstrated, for the first time to our knowledge, by modulation of a saturable-absorber mirror with the light used simultaneously for pumping the gain fiber. Stabilization of a harmonically passive mode-locked fiber laser that generates soliton pulses of 550-fs duration is obtained with this technique. The results indicate a reduction of supermode suppression by more than 55 dB. The timing jitter is improved by 9 dB. (C) 2003 Optical Society of America.