STUDY O F A MOUNTAIN STREAM IN A GRANITIC REGION IN THE NORTH O F PORTU-GAL Macrofauna, fishes and physico-chemical characteristics have been studied in the Olo nver (Douro Basin), placed in the Natural Park of .Alviio, north Portugal. During 1.5 year, every 3 months, samples of macrofauna and water for chemical analyses have been taken in 3 stations along the river. In order to assess fish species and biomass, electrofishing has been made in 2 stations. This stream can be charactenzed as a

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... polluted headstream with low concentrations of organic matter and dissolved salts, and a high stability of the chemical parameters al1 along the year. The chemical data shows no significant difference between the stations. The fish population has a low biomass. Dominant species are Salmo rriltta,fario L. and C'hondo~stromu pol.vlc~pis polylepis S. The macrofauna shows a high diversity and no significant difference betwcen the stations. 177 identification units have been distinguished, and 89 of them are species. Dominant species are Bu~'ti.s rhoduni and Culliurcys humilis in the Ephemeroptera group, Plecrrocncnzia genicitlatu and IIydrop.c-yche siltului in the Tnchoptera one. In Plecoptera Leuctru gr.,fu.\cu is dominant. Stnking differences with data on macrofauna in Spanish parts of Douro Basin are the absence of Baetis gr. alpina in the upstream station, while Tanago ( 1 984) mentions this group for epirhitron, as well as the replacement of Gummuru.s sp. by Pro-a.sel1tr.c gr. co-ua1i.c. unlike the Jalon and Tanago (1982) data.