Carcinomas of the gallbladder with pleomorphic spindle cell histology are unusual. We report such an occurrence in a 47-year-old male with a neoplasm showing both a mixed squamous and a pleomorphic spindle cell pattern. Ultrastructural studies supported the likelihood that this lesion represented a pleomorphic carcinoma and revealed unique paracrystalline structures intimately associated with tonofilaments. For this reason we believe that this and some other previously diagnosed sarcomas and carcinosarcomas of the gallbladder are in reality poorly differentiated carcinomas of either adeno, squamous, or mixed type. This illustrates the need for critical ultrastructural assay to determine the nature of poorly defined and differentiated neoplastic growths.