Mayfly species richness and abundance were investigated at 52 Swedish river rapids. These were either unregulated, or regulated with or without reduced discharge. Sites impacted by regulation had lower mayfly richness and abundances than unregulated reference sites. The relative importance of reduced discharge, daily fluctuations in flow, flow constancy, and the distance to nearest rapid (as a measure of isolation) was evaluated in multiple regression analyses. These indicated negative effects of diel fluctuations on both abundance and richness, whereas flow constancy was favourable for richness. Neither the distance to nearest rapid nor reduced flow were significantly related to mayfly richness and abundance. In total, 26 mayfly species were recorded. Only Baetis rhodani was found at all sites, but another five species were present at more than 40 sites. The strongest effects were found for species within the Heptageniidae. Nineteen of 20 mayfly species present in both the regulated (with unreduced flow) and unregulated reference rapids were on average more common in the unregulated ones. Mayfly assemblage structure was primarily influenced by regional factors and nutrient status, although daily fluctuations in flow together with rapids dimensions also had a significant influence. No rare species appears to be threatened by hydropower regulation though it is conceivable that depressed abundances in regulated rivers indirectly influence predators and periphyton.