SUMMARY.1. Trichopteran populations below a dam on the River Tuntang (Central Java) and on intake screens above the dam consisted almost exclusively ofAmphipsyche meridiana.Larvae on dam screens were in tightly packed aggregations, with greater densities in deeper water, and were predominantly fourth or fifth instars during the period of observation. Below the dam, 40% of larvae were in the first three instars. Larvae at the dam screens were heavier and yielded larger emerging adults which contained more eggs per female.2. Mean net mesh sizes of fifth instar larvae varied greatly from 34×26 μm to 125×106 μm with larger meshes having progressively thicker strands. It is suggested that larger mesh size may be positively correlated with faster flow rates. The dinoflagellatePeridinium, commonly found in the guts of larvae below the dam, was virtually absent in dam screen larvae, possibly due to mesh size differences.3. The behaviour of larvae in aggregations was observed in a laboratory flow channel. After experimental destruction of feeding nets, larvae immediately ingested silk prior to the construction of a new net. Nets were spun in a‘figure 8’fashion, and took between 11 and 30 min to complete. Stridulation between larvae occurred frequently and may be related to the defence of silk reserves within terr