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The present paper is a report on qualitative and quantitative investigations ou the bottom fauna of brooks in the forest district of North Sweden. The work was carried out during the summers of 1952 and 1953 in the vicinity of Bodträskfors. The characteristic physiographical features of the brooks arc: low pH (5.1-5.3), low degree of hardness (0.7 D-degrees). high concentration of iron cations (l.8-3.0 mg/I). A special character of these brooks is the complete absence of a ≫cold stenothermic part ≪. Ecologically all the brooks belong to the Central European ≫ grayling region ≪ because of the wide temperature amplitude. in general the insect fauna consists of boreo-alpine species but in addition endemic high-boreal species are also present. The quantity of bottom fauna increases below the lakes, Furthermore, maximum colonization was caused by the regulations taking place for purposes of timber-floating. Below lakes Trichopteran larvae dominated, on the other hand Simulium larvae attained high density in the cleaned brooks. The brooks cleaned in 1952 show an almost pure colonization by Simulium larvae while those cleaned in 1951 in addition to a dense population of Simulium show increased numbers of rheophilous Trichopteran larvae. The qualitative and quantitative survey of the bottom fauna of the river Spikselån from station 1 (cleaned 1948) shows a gradual transition to Trichopteran larvae. When regulation (cleaning) of a river takes place the velocity of the current increases thus improving the conditions for rheophilous species. Special situations may occur below lakes. The occurrence of Neureclipsis bimaculata and Hydropsyche angustipennis (Trichoptera) is clearly dependent on the variation in current strength. The following current-water biocenoses could be distinguished: 1. Marshy brooks: Biological and physiographical characteristics: low pH, dense vegetation of diatoms (Tabellaria,), low density of Chironomidae and Odonata, 2. Lake-outlet biocenosis. Biological and physiographical characteristics: narrow temperature variations. Because of the variation of the current the species composition varies: Weak current: Neureclipsis himaculata; Spongillidae; Baetidae. Strong current: Hydropsyche angustipennis; Rhyacophila nubila; Simuliidae, Spongillidae. 3. Submerse plant biocenosis. Biological and physiographical characteristics: Wide temperature variation, high O2 content. In strong current: Fontinalis sp. In weak current: Batrachospermum vagum. Typical fauna: Chitonophora aronii; Ephemerella ignita; Paraleptophlebia tumida; Coleoptera; Hydracarina. 4. Rubble biocenosis. Biological and physiographical characteristics: Wide temperature variation; high O2 content, no submerse plants. Typical fauna: Heptagenia dalacarlica; Baetis vernus, Baetis sp.; rarely Coleoptera; Hydracarina. [ABSTRACT FROM AUTHOR]