On December 23, 1999, at 1600 atlantic standard time, a Cessna 208B, N1315A, collided with trees in a heavily wooded area near Adjuntas, Puerto Rico. The air taxi cargo flight was operated by M & N Aviation under the provisions of Title 14 CFR Part 135 with no flight plan filed. Visual weather conditions prevailed at the nearest weather reporting facility at the time of the accident. The airplane sustained substantial damage. The commercial pilot received minor injuries and his passenger received serious injuries. The flight departed Ponce, Puerto Rico, at 1532. While en route to Ponce, the pilot heard a radio transmission that weather conditions in the vicinity of his next destination was "bad." Upon arriving in Ponce, the pilot attempted to telephone Flight Service for a weather briefing and to file a flight plan. After efforts to contact Flight Service failed, the pilot elected to takeoff without filing a flight plan. After a few minutes into the flight, the pilot decided to return to Ponce. Reportedly, the flight encountered deteriorating weather conditions that included heavy turbulence, and low ceilings. While attempting to maintain visual contact with the ground the pilot anticipated executing a left turn to avoid colliding with the high terrain. The airplane collided with trees as the pilot maneuvered the airplane. The post-accident examination of the airplane failed to disclose a mechanical malfunction or a component failure. The pilot did not report a mechanical problem with the airplane. A review of the en route flight charts disclosed that the terrain elevation at the departure point was approximately 24 feet. The terrain elevation at the approximate location of the accident site was 2,500 feet.