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The primary relaxation rate of excited phytochrome is measured by time-resolved laser fluorescence techniques at temperatures between 77 and 300 K in a buffer glycerol mixture. The known multiexponential fluorescence decay of the red absorbing form P r at physiological temperatures reduces to a single exponential below 150 K with a lifetime τ of (1.3 ± 0.2) ns at 77 K. This is unambiguously consistent with the dynamics of two excited states which are in thermal equilibrium with each other and represent the initially excited P r and an excited intermediate of the chromophore during the Z-E photoisomerization.