On April 23, 1997, about 1145 Alaska daylight time, a wheel equipped Cessna 207A airplane, N1785U, owned and operated by Frontier Flying Service, and operating as scheduled commuter flight 526 under 14 CFR Part 135, sustained substantial damage when an aft cargo door opened in cruise flight and struck the right side of the fuselage. The solo commercial pilot was not injured. The flight departed Stevens Village, Alaska, about 1123, and was en route to Fairbanks, Alaska, when the accident occurred. The flight operated in visual meteorological conditions, and a company visual flight rules flight plan was in effect. The cargo door opened about 22 minutes after departing Stevens Village, and about 20 miles northeast of Minto, Alaska. According to a Federal Aviation Inspector who spoke with the pilot, the pilot believed he had closed and latched the cargo door prior to departing Stevens Village. After the door opened in-flight, the pilot elected to continue the flight to Fairbanks, where he made an uneventful landing. An FAA inspector inspected the door and latching mechanism after the accident. He reported he could find nothing wrong with the door or its latching mechanism. Separate conversations with the owner, and the Director of Maintenance of Frontier Flying Service, disclosed a bulkhead, longeron and stringer were damaged when the door struck the side of the fuselage.