Fiber optic dynamic light scattering (FODLS) is fast developing as a technique for sizing particles in concentrated dispersions. Fiber optic systems have the potential to be made into online particle size measuring equipment in process industries. Two different laser light sources were used to study monodisperse polystyrene latex dispersion with different particle sizes and volume fractions. The apparent diffusion coefficients were a function of particle size, volume fraction, and signal to noise ratios. The apparent diffusion coefficient data showed that FODLS probes collective diffusion for smaller particles and self diffusion for larger particles. It was observed that for a polydisperse concentrated dispersion, both self and collective diffusion were probed by FODLS. This means that the apparent diffusion coefficient can be used only as a relative quantity.