Rocks belonging to the Svionian cycle of the Swedish Archean have been mapped and investigated on the small island of Riddarskäret in NE Uppland, Central Sweden. The main rock type is a synorogenic gneiss-granite of granodioritic-quartzdioritic composition, called Uppsala granite. Numerous fragments of amphibolite occur in this granite over the whole area. Sheets of acid supracrustal rocks (leptite) and of banded amphibolites are enclosed in the granite along the planes of schistosity of the latter. The banded amphibolites probably represent basic tuffites and tuffs, possibly also lavas. In the granite a diffuse banding in the direction of strike, possibly derived from highly assimilated supracrustal rocks, is sometimes found. The above rocks are cut by dikes of aplite, granite porphyry and by amphibolite dikes of three different generations, in the directions N80°W, NNW and N70°W, respectively. The youngest rock is a red pegmatite.