Lockheed said the engineering staff found cracks in the rear bulkhead, a major structural part weighing about 300 pounds, of an F-35B ground test plane undergoing fatigue testing, the Fort Worth Star Telegram reported Wednesday.
The F-35B is a short-takeoff-vertical-landing version of the F-35 series designed for the Marines.
The cracks were found after the plane had been subjected to stresses equivalent to about 1,500 hours of flight time.
Structural components of the aircraft are designed to last at least 8,000 hours.
After engineers saw unusual data from test instruments, the cracks were found in a special inspection, the company said.
The latest problem comes just days before an important Pentagon meeting on the program's continuing delays and rising costs.
At an estimated $382 billion, the F-35 is the costliest weapons program ever, the Star Telegram said.