Active mode locking of a Pr3+/Yb3+-doped upconversion fluoride fiber laser with an all-fiber ZnO acousto-optic phase modulator is demonstrated for the first time to the authors' knowledge. Optical pulses of similar to 550-ps duration with a repetition rate of 239 MHz at a wavelength of 635 nm have been generated. (C) 2000 Optical Society of America.