The downstream distribution and larval development of the Plecoptera from a Pyrenean stream were studied upstream and downstream of a hydroelectric power plant with intermittent hypolimnetic releases. During power generation, flow and temperature were the two main environmental factors modified. The downstream distribution of the various taxa reflected both the impact of hydropeaking and the natural zonation. The lowest densities and biomasses were estimated at 700 m downstream from the plant, underlining the prominent role of hydropeaking. Above the plant, Plecoptera larvae showed a constant drift due to accidental dislodgement, and behavioural drift was mainly nocturnal. Below the plant, the flushing action of peaking flows added to this a catastrophic drift, which was even higher since the amplitude between natural flow and peak flow was elevated. The life cycle patterns and the growth rates of the five dominating species (Siphonoperla torrentium, Isoperla acicularis, Perla grandis, Amphinemura sulcicollis, Protonemura beatensis) were similar at the various sampling sites, and only slight differences in hatching and/or emergence periods were recorded. The Plecoptera of the Orie `ge complete their larval development during periods where artificial thermal fluctuations are low and have a minor effect on the populations: from autumn to spring-i.e. when the temperatures remain rather low and during snow melt flood (e.g. S. torrentium, I. acicularis, A. sulcicollis)-or in summer when hypolimnetic releases increase the daily thermal fluctuations but reduce the mean diel temperatures (e.g. P. beatensis). Therefore, under this kind of river regulation, the plecopteran population structure and distribution is mainly governed by hydraulic disturbances.