The flight was cruising at FL330 in visual meteorological conditions. There were no SIGMETS for turbulence in effect. Flight attendants were working in the forward and aft galleys in preparation for meal service when the flight encountered turbulence. According to the flight data recorder, the turbulence consisted of two pulses which were separated by 8 seconds. Each pulse had a maximum value of 1.6 g’s as measured on the vertical axis of the airplane. The seatbelt sign was illuminated. Three flight attendants received minor injuries and a fourth was seriously injured with a fractured ankle. Two passengers in the rear of the airplane whose seat belts were unfastened received minor injuries. Probable Cause: an encounter with unforecast clear air turbulence and the flight attendant’s failure to use the seat belt.