We use fluorescence excitation and two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy to characterize the S-1-S-0 transition of benzo[a]pyrene, B[a]P. In addition, two-color REMPI with a tunable ionization laser provides an improved measurement of the molecular ionization potential, 7.103+/-0.004 eV. Ab initio calculations are performed to determine the vibrational frequencies for the 90 vibrational modes in the S-1 and S-0 electronic states. Our results indicate that the single-excitation configuration interaction (CIS) level of theory provides an excellent representation of excited-state frequencies for this large molecule. Virtually all of the vibronic bands between similar to 380 nm and 397 nm, as well as numerous bands between 373 nm and 380 nm, are assigned with the aid of the calculated vibrational frequencies. In total, 27 of the 61 a’ symmetry vibrational modes and 5 of the 29 a " vibrational modes in the S-1 state have now been assigned.