Passive Q switching of a flash-lamp-pumped ruby laser has been demonstrated with a Dy2+:CaF2 saturable absorber at room temperature. Typically a single Q-switched pulse of 4.5 mJ and 94 ns was observed. At 694 nm, the ground-state absorption cross section and the excited-state absorption cross section of the Dy2+:CaF2 absorber were found to be 1.2 � 10-18 cm2 and 0.9 � 10-18 cm2 respectively, with a four-level slow-relaxing saturable-absorber model.