The news comes just days before an important Pentagon meeting on the program's continuing delays and rising costs.
Lockheed issued a statement saying its Fort Worth engineering staff had 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 cracks were found after the plane had been subjected to the equivalent of about 1,500 hours of flight time. The airplane's structural components are designed to last at least 8,000 hours.
Lockheed said the cracks were found in a special inspection after engineers discovered unusual data from test instruments.