High density excitation effects on fluorescence were studied for the crystals of fluoranthene, chrysene, benzo[g, h, i]perylene, pyrene and pyrene- d 10 and also for the crystal of 1,2,4,5-tetracyanobenzene (TCNB)-hexamethylbenzene (HMB) complex. Relative fluorescence intensity in the shorter wavelength region decreased with increasing excitation density (for chrysene, fluoranthene, and benzo[g, h, i]perylene), and the vibrational structure became diffuse at high density excitation (for fluoranthene and chrysene). The rate constants of the bimolecular quenching and exciton migration were obtained by analysis of the fluorescence decay curves at high density excitation for chrysene, pyrene, pyrene- d 10, and TCNB-HMB. The exciton hopping rates at 295 K were 2.4 × 10 10 s −1 in chrysene and 7.9 × 10 8 s −1 in TCNB-HMB. The differences in the rate between chrysene and fluoranthene (2.2 × 10 9 s −1) and between TCNB-HMB and TCNB-durene (4.2 × 10 9 s −1) are discussed in terms of the coupling intensity between two neighboring oscillators accompanying the transition between ground and the lowest excited singlet state. The difference in the rate and activation energy of exciton migration between pyrene and pyrene- d 10 corresponded to a quasilocalized exciton model.