Nonlinearities photoluminescence, photoconductivity and resonant Raman measurements of fullerenes at high excitation densities are described and the observations are discussed in terms of an optically induced insulator-metal transition in fullerene crystals. The intensity dependence of the transient photoconductive response is reported to show similar nonlinear behaviour. At low intensities, the response decays exponentially within a time of 15nsec. At increased intensities, a second, delayed component emerges and evolves nonlinearly into a long lived component of lifetime 100-200nsec. The system is fitted with a simple three level model in accordance with a Mott-like transition.