Diel flight activity of caddishflies was studied at 4 streams in Northern Thailand. The periods of diel flight activity of the caddishflies were arranged into 4 groups : caddishflies active all night, active at dusk, active around midnight and active at dawn. Species richness was highest at dusk and lowest after midnight. Flight activity (numbers of specimens caught) was most intensive at dusk and lowest toward dawn. To obtain a valid estimate of species diversity it is necessary to operate the light trap throughout the night.