The activity and dynamics of a simple, single subunit enzyme, the xylanase from Thermotoga maritima strain Fj SS3B.1 have been measured under similar conditions, from -70 to +10 °C. The internal motions of the enzyme, as evidenced by neutron scattering, undergo a sharp transition within this temperature range; they show no evidence for picosecond-timescale anharmonic behaviour (e.g. local diffusive motions or jumps between alternative conformations) at temperatures below -50 °C, whereas these motions are strongly activated at higher temperatures. The activity follows Arrhenius behaviour over the whole of the temperature range investigated, -70 to +10 °C. The results indicate that a temperature range exists over which the enzyme rate-limiting step is independent of fast anharmonic dynamics.