Forward light scattering has been a popular technique for measuring the fractal dimension of aggregated or flocculated particles. The method relies on the measurement of the slope of scattered intensity versus scattering vector (q) on a log-log plot. In the Rayleigh-Gans-Debye approximation, the negative of the slope has the value of the mass fractal dimension of the aggregates. In this work, simulation results were used to test the idea that Fraunhofer diffraction can be used to describe scattering from fractal aggregates. Polystyrene latex of diameter 4.9 mu m was aggregated in a forward light scattering instrument, and the fractal dimension was determined independently by the volume obscuration method. Comparison of the experimental data with simulations showed poor agreement. It is suggested that multiple scattering within the aggregate structure, ignored in the usual Fraunhofer formulation, is significant. (c) 2005 Elsevier B.V. All rights reserved.