The captain initiated descent from flight level (FL) 290 and illuminated the “fasten seatbelts” sign. Upon reaching FL 240, the air began to get “choppy.” As they descended below FL 240, the flight attendants were notified to clean up and be seated. A passenger announcement was made at the same time including possible turbulence. Approximately FL 230, the flight attendants were notified to all be seated. At FL 220 they encountered severe turbulence for about 5 seconds. Two or 3 minutes later, they encountered severe turbulence again, for about 5 seconds. On the way to her seat, one of the flight attendants stopped because of a passenger who had not fastened his seat belt. While she was instructing the passenger, the aircraft encountered severe turbulence and she was thrown about the cabin. The “A” flight attendant next notified the flight crew that the “B” flight attendant had been injured. The captain told the “A” flight attendant to help put the “B” flight attendant in her seat and then to return to her own seat as soon as possible. Air traffic control was notified of the severe turbulence and that medical assistance would be needed upon their arrival. Company flight operations in Fresno was also informed of the situation. Upon arrival at the gate, all of the passengers deplaned without further incident. Paramedics boarded the aircraft and attended to the “B” flight attendant. Subsequent x-rays revealed that she had sustained a fractured ankle. Probable Cause: the failure of the passenger to fasten his seat belt without the necessity of being instructed by the flight attendant.