Larvae of Geometridae (Lep.) attacked the birch wood (Betula pubescens Ehrh.) in the mountain slopes near Sogndal, Sogn, in I966 and I967. Collecting of larvae were made in I967 by sweep-netting on five stations at different altitudes up to timber line at 900 m a.s. The maximum density of Operophthera sp., at about 300 m a.s., was estimated to be about five times higher than the maximum density of Oporinia autumnata Bkh. just below the timber line. A horizontal, brownish belt, where the trees were heavily defoliated, extended from approximately 250 to 350 m a.s. The trees near the timber line were not so heavily defoliated. The least defoliated trees were found at about 650-700 m a.s., in the overlapping zone of the two geometrid species. Operophthera sp. was found at all altitudes, while O. autumnata occurred only in the upper half of the birch wood. Erannis aurantiaria Hb. and E. defoliaria Cl. also occurred at all altitudes. The former preferred the lower part of the birch wood, and was always present in densities lower than O. autumnata's highest density. Only a few larvae of E. defoliaria were found, without showing any altitudinal preference.