Cinematographic observations have been made of crack propagation under well‐defined boundary conditions in rubbers at speeds up to 30 m/sec. The fracture‐markings showed resemblances to those obtained with metals, plastics, and glass, and could be related directly to the corresponding speed of fracture‐propagation. In close analogy with Schardin's observations on glass, a noncrystallizing rubber (GR‐S) showed a mode of crack propagation in which the fractured surfaces were visually smooth and the speed was about one quarter of the speed of longitudinal elastic waves. A crystallizing rubber (natural rubber) did not show this mode of propagation under the present test conditions. It is suggested that the modes of solid fracture can be usefully classified in three categories: (i) slow propagation, generally with smooth surfaces, obtained by careful control of the boundary conditions; (ii) propagation at intermediate rates with rough surfaces, involving correspondingly greater energy consumption; and (iii) fast propagation with smooth surfaces, the rate of propagation being limited by the speed of elastic waves in the material in accord with Mott's theory.