The ultrafast fluorescence dynamics of solutions of the chromophore responsible for emission from the green fluorescent protein are measured by fluorescence up-conversion, Decays are non-exponential but well fit by a sum of two-exponentials. All decays have a prompt rise time. The two decay times are approximately independent of wavelength, but their weights are wavelength dependent, in a manner consistent with a spectral narrowing with time. The longer decay time has a weak dependence on medium viscosity. (C) 2002 Elsevier Science B.V. All rights reserved.