Using a blue copper protein, plastocyanin, we have studied protein dynamics associated with optical excitation and radiationless deactivation (tau = 270 fs) of its charge-transfer state by ultrafast pump-probe spectroscopy. A coherent Ion frequency (33 cm(-1)) vibrational mode with its phase shifted from those of the other localized Raman modes was observed. The observed coherency of this low frequency mode appears via the excited-state deactivation, rather than arising merely from impulsive stimulated Raman. This mode most probably can be attributed to the delocalized mode including the protein skeletal motion.