Light-activated proton translocation in halobacteria is driven by photoisomerization of the retinal chromophore within the membrane-bound protein bacteriorhodopsin. The molecular mechanism of this process has been widely debated due to the absence of structural information on the time scale of the reactive dynamics (the initial 0.1-1 ps). Here we use tunable femtosecond stimulated Raman spectroscopy to obtain time-resolved resonance Raman vibrational spectra of bacteriorhodopsin's key J and K photoisomerization intermediates. The appearance of the J state is delayed by similar to 150 fs relative to the zero of time and rises after this dwell with a 450 fs time constant. The J state is characterized by a 16 cm(-1) red-shifted C=C stretch, which blue shifts by 5 cm(-1) coincident with the rise of the K state. The delayed 3 ps rise of the C-15-H HOOP mode with enhanced intensity in K reveals the appearance of strain near the Schiff's base once the 13-cis configuration is fully formed. The delay in the initial appearance of J is assigned to nuclear dynamics on the excited state that precede the formation of the proper geometry for reactive internal conversion.