On August 25, 2001, at 1450 eastern daylight time, a Beech 55, N521U, landed with left main landing gear up at the Mayport Airport, in Jacksonville, Florida the training flight was operated by the Navy Flying Club under the provisions of CFR 14 Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The certified flight instructor and his student were not injured. The airplane received substantial structural damage. The flight departed Jacksonville, Florida, at 1405. During the seventh touch and go landing, the left main landing gear failed to extend. According to the pilot, he completed the landing check and verified that the landing gear cockpit indicators light showed a down and locked position. The pilot continued the approach and the airplane touched down with the left wheel retracted. The subsequent examination of the airplane revealed that the left main landing gear extension rod was bent. The examination also revealed that the uplock rollers were maintained in accordance with required maintenance procedures and performed without discrepancies during the post-accident functional check. The landing gear extension rod was replaced, and a successful functional check of the landing gear system was accomplished. According to the flying club assistant chief pilot, this airplane was used for training, and the origin of the extension rod deformation was not determined. Since, the landing gear indicator system only shows the position of the landing gear motor, the pilot had no means of verifying when the main and nose gear assemblies were in the extended and locked positions.