Nine thymomas, a normal adult thymus, and tissue culture of one thymoma were studied ultrastructurally. The histologic types of thymoma included lymphocytic, epithelial, mixed and spindle cell varieties. Two of the tumors were invasive and one was associated with myasthenia gravis. Despite the histologic dissimilarities, the complex anatomic interrelationships involving lymphocytes, epithelial cells, and blood vessels as seen in the thymus tended to be preserved in all but one of the thymomas. The exception was the spindle-cell thymoma which contained only rare lymphocytes and simpler vascular structures. The findings include variations in the frequency of demosomes and cytoplasmic fibrils of the epithelial cells, the occasional presence of lymphocytes within the laminated venules, and the unusual finding of a gland-like structure in one of the tumors. Correlations between ultrastructural changes and clinical behavior could not be made.