The stridulatory apparatus of eight species of 5th instar caddis larvae of the family Hydropsychidae were compared using the scanning electron microscope. This revealed the presence of secondary structures associated with each main ridge of the file in some species. Slight differences occur in the number of ridges which form the plectra. The sounds produced by two coexisting species of hydropsychid larvae were recorded and temporal and frequency analysis carried out. These sounds are found to extend far into the ultrasonic range and appear to show only minor interspecific differences. The occurrence of stridulatory behaviour during territorial defence and its significance in relation to the ecology of the larvae is discussed. Some physical parameters affecting sound reception in the near- and far-field are presented and this leads to speculation on the existence of a possible hearing mechanism.