Crack propagation is the basic mechanism of materials failure. Experiments on dynamic fracture in brittle amorphous materials have produced results＇ that agree with theoretical predictions for single-crack motion at very low velocities. But numerous apparent discrepancies with theory have been observed(2-4) at higher velocities. In particular, the maximum crack velocities attained in amorphous materials are far slower than the predicted asymptotic value, v(R) (ref. 3). Beyond a critical velocity, v(c) approximate to 0.4v(R), an intrinsic instability has been observed(5) in which a multiple-crack state is formed by repetitive, frustrated micro-branching events. These cause velocity oscillations and may explain the apparent anomaly. Here we report measurements of dynamic fracture in a brittle, amorphous material that are in quantitative agreement with the theoretical single-crack equation of motion, from the initial stages of propagation up to v(c). Beyond v(c), agreement breaks down owing to the appearance of the multiple-crack ensemble. But in this regime, the micro-branching process can momentarily produce a single-crack state which instantaneously attains its predicted single-crack velocity, for velocities up to 0.9v(R). Our results therefore confirm the validity of the single-crack continuum theory of elastic brittle fracture even in the dynamical regime where the crack morphology is complex.