Over a two-year period, 77 suspected oak wilt centers were examined in 7 Texas counties. Fungal mats, formed by the oak wilt pathogen Ceratocystis fagacearum, were discovered under the bark of diseased Spanish oaks at 12 of the sites. Mats were not found on live oaks. Five types of symptoms were observed on 138 diseased trees selected for sampling and laboratory isolation of C. fagacearum. Veinal necrosis and tipburn of leaves occurred separately or in combination on 51% of the diseased live oaks. These symptoms were reliable predictors for isolation of C. fagacearum. Typical wilting was found only on diseased Spanish oaks. Interveinal chlorosis, non-specific decline symptoms, and various other combinations were also found on trees from which C. fagacearum was isolated. In one 36-acre oak wilt center, 45% of the oaks surveyed were dead or dying. The remaining trees, in various states of health, were scattered throughout the center. These observations have improved early detection and diagnosis of oak wilt in live oaks and contributed to the conclusion that oak wilt is the most serious oak disease in Texas.