AbstractThe benthic invertebrate fauna of the lowland part of the Aurland River was investigated in 1966–7 over a full year cycle. The watershed was built out for hydropower production during 1979–83. In this study the life cycle and production ofBaetis rhodanifrom pre‐regulated conditions (1966–7) was compared with two types of post‐regulation streams (1988–9). Before regulation the water flow was high during May‐June and in autumn, combined with heavy rainfall. The lowest flow was in winter and early spring. The pre‐regulated temperature was low in winter, increasing from April to a maximum in August (10–14°). After regulation one part of the river received reduced flow and increased summer temperatures (upper part) and one received hypolimnion release and reduced summer temperatures (lower part). In the upper part the density ofB. rhodaniincreased between 10 and 20 times. The reason for this seems to be increased temperature, reduced accidental drift of larvae and increased amount of stored organic material on the bottom. In the lower part the density increase was two to five times and the production two times higher after regulation. The lower increase in production than density was due to a much higher proportion of small larvae.B. rhodanihad mainly an univoltine life cycle before regulation, but a small part of the population was bivoltine. After regulation the species was univoltine in the lower part. In the upper part the life cycle was about one month faster, but no clear indication of bivoltinism was seen. The absence of the second generation is due to low temperatures and probably a lack of signals connecte