Several brittle thermoplastic materials, including polystyrene, SAN, and PMMA, were tested in tension using a spectrum of testing rates up to 15,000 in./min (630 cm/sec). At test velocities below 7000 in./min (294 cm/sec), the materials behaved “classically” with strength properties increasing, elongation decreasing, and single fractures generally forming. At rates between 7000 and 9000 in./min (294 and 378 cm/sec), a transition from classical to stress wave-initiated fracture was observed. The mechanical properties, the nature of the fracture, and the nature of the crazing preceding fracture were shown to be profoundly affected when the materials were strained at rates sufficient to initiate stress wave fracture. Under these conditions, the apparent load-carrying capabilities of the materials were reduced by an order of magnitude.