We report the observation of the biexciton in semiconducting single-wall carbon nanotubes by means of nonlinear optical spectroscopy. Our measurements reveal the universal asymmetric line-shape of the Fano resonance intrinsic to the biexciton transition. For nanotubes of the (9,7) chirality, we finnd a biexciton binding energy of 106 meV. From the calculation of the xi(3) nonlinear response, we provide a quantitative interpretation of our measurements, leading to an estimation of the characteristic Fano factor q of 7+-3. This value allows us to extract the rst experimental information on the biexciton stability and we obtain a biexciton annihilation rate comparable to the exciton-exciton annihilation one.

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