The dynamic response characteristics of 0.4-mm-diameter glass rod thermistors, which are used frequently in fast-response CTD and microstructure instruments, have been determined over the speed range of 0.08--3.8 m/s. The spectral transfer function was obtained by firing the probes through a thin plume an then comparing the power spectrum of the shot data with that of a slow-speed map of the plume. The spectral transfer functions are described accurately by a two-pole filter for frequencies less than 25 Hz, although a single-pole form is an equally good representation for less than 10 Hz. The frequency of the 1/2-power (3-dB) attenuation point scales as U1/3. Although the spectral forms an velocity scaling of the responses of the different probes tested are the same, the levels show significant differences due to irregularities in the glass coatings around the thermistor material.