On June 28, 1998, at 1915 hours Pacific daylight time, a Cessna 180H, N4803E, sustained a collapsed left main landing gear during rollout following landing on a dirt strip at the Ocotillo Wells, California, airport. The airplane, owned and operated by the pilot under 14 CFR Part 91, sustained substantial damage. The airline transport pilot/owner and pilot rated passenger were not injured. Visual meteorological conditions existed for the personal flight and no flight plan was filed. The flight had originated from Fallbrook, California, at an unknown time, with an intermediate stop at Gillespie Field, San Diego, California, to pick up the passenger before continuing on to Ocotillo Wells. The pilot stated that they arrived at Ocotillo Wells, a dry lake bed, at approximately 1915. He stated that on touchdown he heard a "snap sound." On the landing rollout, at 20 knots, the left main landing gear turned under the fuselage and departed the airplane. The pilot reported that the airplane made a slight turn to the right before falling on the left wing. The passenger noted that no discrepancies were noted with any portion of the flight; however, right after touchdown he heard a snap or pop sound, then the airplane rolled to the left. Review of the airplane logbook by a Safety Board Investigator revealed that the last annual maintenance was performed on July 24, 1997, with no discrepancies noted. The landing gear was inspected by a Federal Aviation Administration (FAA) inspector. He noted that the landing gear attach bolt was distorted, and the threads on the bolt and nut were stripped. He was able to view through the access hole the left-hand attachment hole for the landing gear attachment bolt. He noted that the attachment hole was "slightly elongated." The FAA inspector reported that the nut was not found attached to the landing gear attach bolt, but at the bottom of the airplane. The airplane was recovered from the accident site by Aircraft Recovery Services, Compton, California, and interviews were conducted with the retrieval crew. According to their statements, the runway was a dirt surface that did not have ruts, potholes, or other defects. While working on recovery, the crew examined portions of the runway and did not observe any unusual tracks or markings on the surface until just before the resting point of the airplane. According to a Cessna representative, the landing gear is visually inspected annually or the 100-hour inspection. The inspection includes checking for cracks, metal distortion, loose or missing rivets, screws, and bolts and evidence of excessive loads. (See appended file). Federal Aviation Regulation Part 43(e) states that at the annual or 100-hour inspection; linkages, trusses and members will be inspected for undue or excessive wear fatigue and distortion.