The dependence of fluorescence intensity on the wavelength of exciting radiation has been recorded for anthracene vapour at 0·09 mm and 170°Cover the range 230–366 mμ. It is shown that whereas the photon cascade theory cannot account for the much higher intensity excited in the 250 mμband than excited in the band at 363 mμ, this behaviour is precisely that expected if the fluorescent state is populated by internal conversion following excitation in the short-wave band.