We report results on line widths in the fluorescence excitation spectra of single pentacene molecules in p-terphenyl crystal flakes for the O-1 and O-2 sites at 1.85 K. A setup for the measurement of fluorescence excitation spectra of single impurity molecules with an overall quantum efficiency of about 0.1% has been designed. Spectra obtained with a signal-to-noise ratio up to 10:1 were recorded at different laser excitation intensities in the range from 1 to 5 mW/cm(2). The line widths measured for different pentacene impurity molecules and extrapolated to zero probing intensity show remarkable variation in the range from 5 to 15 MHz. A reason why some lines are narrower than the lifetime-limited one (8 MHz) is discussed.