A description is given of the construction and operation of a cross-correlation phase and modulation fluorometer which uses the harmonic content of a high repetition rate mode-locked laser as the excitation source. A mode-locked argon ion laser is used to synchronously pump a dye laser. The pulse train output from the dye laser is amplitude modulated by an acousto-optic modulator and then frequency doubled with an angle tuned frequency doubler. Experimental data are provided to show use of this high frequency cross-correlation phase-modulation fluorometer for the determination of fluorescence lifetimes and rotational motions of tryptophan in solution and in proteins.