A threshold for the production of two fluorescence bands in CdS under electron bombardment at - 196\ifmmode^\circ\else\textdegree\fi{}C has been observed at 290 kev. This corresponds to a maximum energy transfer of 7.3 ev to the cadmium atom. The fluorescence bands produced are at about 6050 A and 1.03 \ensuremath{\mu}. An overlay experiment in which cadmium atoms diffused into the CdS lattice in interstitial positions produced the 6050 A band which is therefore believed to be a result of cadmium interstitial atoms. The 1.03 \ensuremath{\mu} band is believed to be a result of cadmium vacancies. This latter band can also be produced by heating undoped CdS crystals to 200\ifmmode^\circ\else\textdegree\fi{}C in air, argon or vacuum.