Examines the view that dominance is the appropriation of potential niche space of certain subordinate species by other, dominant species, and so can be manifested most clearly only within the same trophic level. Field studies were undertaken using the caddisfly family Hydropsychidae in the Rio Duraton, N Spain, where Hydropsyche pellucidula was dominant. Interspecific competitive interactions did seem to be the main factor in determining species persistence in hydropsychid guilds. -P.J.Jarvis