On December 15, 1994, approximately 1050 mountain standard time, a Beech 1900D, N39ZV, was substantially damaged when the aft cargo door opened during takeoff at Farmington, New Mexico. There were no injuries to the two pilots and 13 passengers. Instrument meteorological conditions prevailed. According to Mesa Airlines, the airplane was operating as United Express flight 7549, a regularly scheduled domestic passenger flight from Farmington, New Mexico, to Durango, Colorado. As the airplane was rotated for liftoff, the aft cargo door opened and a piece of baggage fell onto the runway. This was corroborated by witnesses. The pilot elected to continue the takeoff, then returned for an uneventful landing. Post accident inspection disclosed damage to the cargo door frame, pressure vessel, and dorsal fin. There was no evidence of failure or malfunction of the cargo door locking system. In the pilot/operator report, the captain said the airplane took off and climbed to an altitude of 100 to 200 feet. When he tried to pressurize the airplane, he heard a loud noise and the cargo door annunciator light illuminated. During the interview the crew stated that prior to takeoff the baggage door light was not illuminated and that it illuminated after becoming airborne. After landing they kept electrical power on and tested the door, when the door was open the light illuminated and when it was closed it did not.