In extremely purified naphthalene crystals triplet exciton lifetimes up to 500 msec at room temperature were measured. The triplet annihilation coefficient is measured from an analysis of the light intensity dependence of delayed fluorescence as γ = 3.5 · IO-12 cm3 sec-1 at 300 °K and at 135 °K. The quantum yields in the weak excitation limit at room temperature are ηDF = (3 ± 0.6) · 10-3 for delayed fluorescence and ηp = (1.2 ± 0.3) • 1O-3 for prompt phosphorescence, both after triplet excitation. The phosphorescence spectra of naphthalene and perdeuteronaphthalene crystals are analyzed between 300 and 50 °K. The line width decreases from 270 cm-1 at 300 °K down to 42 cm-1 at 60 °K.