Benthic animal communities in streams with varying environmental conditions in a national park in northern Swedish Lapland were analysed. Dominant taxa were Ephemeroptera, Plecoptera, Trichoptera and Diptera Simuliidae. Northeasterly or very widely spread species make up the fauna. Intrariverine distribution patterns were found to differ between taxa, and great variations of standing crop values between adjacent sampling sites were the rule. Day-to-day fluctuations of drift rate were unsynchronized between taxa. Some ecological properties of Brachyptera risi (Plec.) are discussed. Some lake shores and streams affected by hydroelectric exploitation appeared virtually devoid of macroscopic animals.