The asymmetrical valleys of Eastern Hokkaido, Northern Japan, are discussed in this paper and it is concluded that they were formed by both periglacial mass-movements and fluvial processes under the permafrost environment of the Last Glacial age.<br> The Konsen Genya Plain of Eastern Hokkaido consists of hills, uplands, and fans com-posed of unconsolidated deposits (Fig. 1). The Kucharo and Mashu volcanoes are located to the west of the Konsen Genya Plain and have ejected great volumes of pumice, ash, and pyroclastic flows. The age of formation of some of the geomorphic surfaces in the region have been determined by tephrochronology using ¹⁴C dating (Fig. 2). In the Konsen Genya Plain, many streams flow almost parallel to each other in a west-east direction, forming shallow flat-bottomed valleys.<br> Asymmetrical valleys are distributed in various parts of the Konsen Genya Plain (Fig. 4). However, intense asymmetry of the valley slopes, with the steeper slopes oriented towards the north, is best developed on the Kamishunbetsu surface which is a fan composed of sand and gravel. Since the fan was formed between 30, 000 and 17, 000 years ago, the valleys dissecting the fan were formed during the Last Ice-Age.<br> The asymmetrical valleys in the study area of the Kamishunbetsu surface were mapped from aerial photographs (Fig. 6). Asymmetry was well developed in the Shunbetsu, Tokotan, and their tributary valleys (Fig. 7). The mean inclinations of the slopes in the asymmetrical valleys were estimated from maps (scale, 1:5, 000) and the maximum angles and transverse profiles were measured in the field by tape and Brunton pocket transits. The results obtained several variations in the asymmetrical features of the valleys from the head to the downstream region (Figs. 8, 9).<br> The asymmetrical features are less distinctive at the heads of the valleys, where both slopes are gentle but the north-facing slopes are slightly steeper than the others (e. g. Fig. 9, A-1). Asymmetry develops progressively towards downstream as the north-facing slopes gradually increase in angle and the south-facing slopes maintain constantly low angles. At a distance of 5_??_6km from the head, the valleys attain maximum asymmetry. The angles of the steeper north-facing slopes reach l5_??_20°, while the southfacing gentle slopes have uniform angles Isse than 7_??_8°. The cross profiles show a sharp contrast between the steep convex form of the north-facing slopes and the long smooth form of the south-facing gentle slopes. Downstream from the zone of maximum asymmetry, the gentle south-facing slopes increase in angle in response to downcutting and lateral erosion by the streams. Steep meander bluffs are produced on both slopes far down the valleys, so that a symmetrical valley form is established there. The apparent pattern of evolution of the valley profile thus suggests that the steep north-facing slopes are a direct result of the erosive action of the streams.<br> It has been assumed that the landscape of Eastern Hokkaido represents a “fossil” penglacial one. This idea has been supported by the finding of widespread evidences of fossil periglacial phenomena such as ice-wedge casts. The asymmetrical valleys were apparentry formed under the permafrost environment of the Last Glacial age since the slopes of the valleys are covered by Holocene tephra.<br> These facts indicate that the stronger erosion occuring at the foot of the north-facing slopes may have been related to microclimatic differences caused by the different attitudes of the two slopes. The microclimatic differences, such as in the intensity of insolation and the extent of snow-drifts driven by northwest winds, accentuated differences in the degree of development of solifiuction processes.