AbstractTemporal alterations in the structure of hydropsychid guilds were studied along a regulated stream (Río Duratón, northern Spain) to examine the role of interspecific competitive interactions and environmental perturbations in determining species persistence. Discharge fluctuations of hypolimnial waters with a significant oxygen deficit were produced daily by Burgomillodo Dam. Under unregulated conditions (upstream sampling site), it is assumed that the coexistence of competing hydropsychid species is a function of species dominance (d') and resource limitation (R), as major causes of interaction strength among competitors, and the niche overlap (O) as a major cause of connectance among competitors,d' tends to increase with the number of subordinate species. Thus, and according to May's model of stability, a hydropsychid guild will tend to be unstable (by competitive displacement) ifd'R(SO)0.5>1, whereSis the number of competing species in the guild. The productd'R(SO)0.5is used as a competition coefficient (β′). To facilitate the calculation of β′, the values ofRand0are assumed to be 1 and 0.5, respectively. Riffle macrobenthic surveys were undertaken in June and December 1987, and in June 1988. Upstream from the dam, values of β′ were 0.599, 1080 and 0.656, and values ofSwere 7, 7 and 6. Downstream from the dam, β′ was less than unity during all sampling surveys, butSdecreased at all downstream sampling sites (mainly just below the dam). Total density and total biomass were significantly higher downstream than upstream (except just below the dam).Hydropsyche pellucidulawas dominant upstream, whereasH. siltalaibecame dominant downstream. Temporal variations in individual weights ofH. bulbiferaandH. exocellatadownstream were different from those upstream. This fact is interpreted as a phenomenon of character displacement. It is concluded that interspecific competitive interactions upstream and environmental perturbations downstream determined species persistence in hydrop