AbstractI. An experimental study on the larvae of Hydropsyche angustipennis, H. siltalai, H. nevae, and H. pellucidula showed that when forced to leave their retreats the larvae would enter any retreat of suitable size, regardless of what species had built it and whether it was occupied or not. If the retreat entered was occupied, the larvae fought vigorously about the ownership of the retreat. 2. When defending their retreats against intruders of various congeneric species, larvae of H. angustipennis, H. siltalai, and H. nevae won more than 90% of the encounters, and all the species were about equally successive as defenders. As intruders, in contrast, H. angustipennis and H. siltalai were clearly superior to H. nevae. 3. Larvae of H. angustipennis and H. nevae were observed to stridulate in about 50% of the encounters, but larvae of H. siltalai stridulated in only about 20% of the fights. In more than 97% of the observed cases it was the defending larva which stridulated, and the stridulations were most frequent during fights between conspecific individuals, and when the intruder was larger than the defender. Intruders stridulated only occasionally, and only if they managed to force their way into the retreat, i.e., they defended the position attained. H. angustipennis was clearly the most active species to stridulate as intruder. 4. In general, the percentage of fights won by the defender decreased as the size of the intruder increased. However, if the defender stridulated, the decrease in the winning percentage was insignificant, but if the defender did not stridulate the decrease was considerable. Thus, stridulation clearly increased the defender's chances of winning a fight, especially against a larger intruder. 5. Morphologically the stridulatory apparatus of the four species differed little from each other. Correspondingly, the structure of the individual sound bursts was practically the same in all the species, but in the grouping of the bursts the species were rather different. Further, variation in numbers of sound bursts per set and numbers of sets per stridulatory period correlated to some extent with the species and size combination of the combatants. 6. The structural similarity of the sound bursts in the four species and the wide frequency band of the sound, together with the fact that stridulation could be provoked very easily by nonspecific artificial stimuli, suggest that the sounds are part of the defense behavior of the larvae and may be classed as protest sounds. Whether they affect other than congeneric intruders remains unknown. 7. Observations on the larvae during their responses to various disturbances indicated that they sensed vibrations carried by their nets and retreats, but not those carried by the water. From this, it is inferred that peculiar plume-like hairs present in large numbers on the anteroventral sides of the basal segments of the fore legs are the vibration receptors of the larvae.