Ultrahigh-pressure dependence of the excited-state proton transfer (ESPT) in the wild-type green fluorescence protein (wt-GFP) in D2O was measured using steady-state and picosecond time-resolved fluorescence spectroscopies. The proton dissociation rate of the chromophore is almost insensitive to a pressure increase up to about 1.1 GPa. In contrast, the diffusion-limited geminate recombination kinetics is strongly affected by pressure, decreasing the effective dimensionality of proton diffusion as pressure increased. The GFP beta-barrel structure sustains the high pressure and unfolds only at P > 1.5 GPa. Published by Elsevier B. V.