Dilute mixed molecular crystals of pentacene in naphthalene were prepared by cosublimation of the two compounds. Single pentacene molecules could be detected in the thin platelets by fluorescence excitation spectroscopy. By analyzing the exponential decay of the fluorescence intensity autocorrelation function for a single molecule in the microsecond time regime we could determine the population (k(23)) and depopulation (k(31)) rate of the metastable triplet state. The increase of the population rate k(23) by a factor of similar to 4 when compared to pentacene in p-terphenyl is responsible for the weaker fluorescence signals of single pentacene molecules in naphthalene crystals.