Abstract: A polytetrafluoroethylene composite sheet having a tensile strength of at least one megapascal and comprising particulate material distributed in a matrix of interentangled polytetrafluoroethylene fibrils. The composite sheet is prepared by blending the particulate material with polytetrafluoroethylene aqueous dispersion in the presence of sufficient lubricant water to exceed the absorptive capacity of the solids yet maintain a putty-like consistency, subjecting the putty-like mass to intensive mixing at a temperature between 50.degree. C. and 100.degree. C. to cause initial fibrillation of the polytetrafluoroethylene particles, biaxially calendering the putty-like mass to cause additional fibrillation of the polytetrafluoroethylene particles while maintaining the same water content and drying the resultant sheet.