Corky root symptoms caused by Pyrenochaeta lycopersici were observed on the roots and stem base of tomato plants in Korea. Symptoms on infected plants typically appeared as stunting and generally lacking vigor, and infected plants die back from the foliage tips after fruits have set. Brown lesions appearing with bands around the roots were characteristic symptoms of the disease. The lesions become swollen and cracked along the length of the root with corky appearance. Based on cultural and morphological characteristics, the fungus from the diseased plants was identified as Pyrenochaeta lycopersici. Pycnidia were solitary, globose to subglobose, brown to black, darker around the neck region, and measured 173-215 mm in diameter with septate setae up to 102-132´6.5 mm. Conidia were hyaline, unicellular, and 4.2-4.7´1.5-2.0 mm long. Optimum temperature for mycelial growth of the P. lycopersici isolates ranged from 20˚C to 25˚C. Fifteen isolates of P. lycopersici were tested for pathogenicity to susceptible and tolerant cultivars of tomato plants by artificial inoculation. Three isolates of P. lycopersici induced typical corky root discoloration on susceptible tomato cultivars but not on tolerant tomato. This is the first report in Korea of tomato corky root disease caused by P. lycopersici.