SummaryA serological technique was used to demonstrate that one species of trichopteran and three species of plecopteran nymphs were feeding on stream‐dwelling triclads. Observations on streams in Caernarvonshire, North Wales, suggested that, in general, these predators were not typical members of the fauna at springheads, where triclads were frequently abundant. In contrast, further downstream the presence of predators was considered to be one of a number of factors which contributed to lower population densities of triclads. However, two small stony streams were encountered where the plecopteran predator Dinocras cephalotes occurred in close proximity to stream‐dwelling triclads from the springhead downwards. In one, both predators and prey were found at all stations but highest triclad numbers were recorded where predators were few. In the second stream the population density of the predator was higher, and here, triclads were very scarce on the undersides of stones and were almost entirely confined o t patches of moss. It appears that the predators may be both limiting the micro‐distribution of the triclad and also perhaps feeding selectively on larger individuals since mature triclads were rare, except at the sprin