Structural fluctuations in proteins on the picosecond timescale have been studied in considerable detail by theoretical methods such as molecular dynamics simulation, but there exist very few experimental data with which to test the conclusions. We have used the technique of inelastic neutron scattering to investigate atomic motion in hydrated myoglobin over the temperature range 4-350 K and on the molecular dynamics timescale 0.1-100 ps. At temperatures below 180 K myoglobin behaves as a harmonic solid, with essentially only vibrational motion. Above 180 K there is a striking dynamic transition arising from the excitation of nonvibrational motion, which we interpret as corresponding to torsional jumps between states of different energy, with a mean energy asymmetry of 12 kJ mol-1. This extra mobility is reflected in a strong temperature dependence of the mean-square atomic displacements, a phenomenon previously observed specifically for the heme iron by Mössbauer spectroscopy, but on a much slower timescale (10(-7) s). It also correlates with a glass-like transition in the hydration shell of myoglobin and with the temperature-dependence of ligand-binding rates at the heme iron, as monitored by flash photolysis. In contrast, the crystal structure of myoglobin determined down to 80 K shows no significant structural transition. The dynamical behaviour we find for myoglobin (and other globular proteins) suggests a coupling of fast local motions to slower collective motions, which is a characteristic feature of other dense glass-forming systems.