This paper considers photon correlation spectroscopy of spontaneous fluctuations in the number of particles in a small scattering volume containing interference fringes formed by counter-propagating laser beams. The ‘fringe number fluctuation’ contribution to the correlation function of the scattered intensity is predicted to decay exponentially in time. The particle diffusion constant can be obtained directly from the measured decay time and the easily calculated fringe spacing. Experiments were performed on both quasi-elastic and fluorescent scattering from dyed polystyrene particles of diameter 0·109 μm in aqueous suspension. For the quasi-elastic scattering reasonable agreement with theory was found. Some difficulties were encountered with the fluorescent scattering; nevertheless, the experimentally derived value of the diffusion constant was close to the expected value. Thus the fringe number fluctuation technique appears to provide a useful modification of the usual fluorescence correlation spectroscopy.