Recent advances in fluorescence excitation spectroscopy have produced significant increases in detection sensitivity for single pentacene molecules in crystals of p-terphenyl at low temperatures. With the increased signal to noise ratio, we have observed two classes of pentacene behavior. While some pentacene defects have fixed, lifetime limited Lorentzian excitation line shapes (7.8 MHz FWHM), other pentacene defects exhibit spontaneous shifts in resonance frequency of up to 100 MHz. Residence times between spectral jumps are on the order of seconds to minutes at 1.5 K. Increasing the temperature from 1.5 to 6 K appears to produce a faster-than-linear increase in jump rate with temperature; we suggest single pentacene defect spectral diffusion is due to thermally assisted structural changes in the p-terphenyl environment. © 1992.