Photoisomerization of a protein bound chromophore is the basis of the light sensing and signaling responses of many photoreceptors. Z-to-E photoisomerization of the Pr Cph1 Delta 2 phytochrome has been investigated by polarization resolved femtosecond visible pump-infrared probe spectroscopy, which yields structural information on the Pr excited (Pr*), Pr ground, and lumi-R product states. By exhaustive search analysis, two photoreaction time constants of (4.7 +/- 1.4) and (30 +/- 5) ps were found. Ring D orientational change in the electronic excited state to the transition state (90 degrees twist) has been followed in real-time. Rotation of ring D takes place in the electronically excited state with a time constant of 30 +/- 5 ps. The photoisomerization is best explained by a single rotation around C-15=C-16 methine bridge in the Pr* state and a diffusive interaction with its protein surrounding.