The physical and chemical characteristics of the dimictic, ultraoligotrophic, and subalpine Lake Atnsjøen are described based on data from the period 1985-2001. The temperature stratification of the lake is weakly developed, with the thermocline at about 10 m depth. The surface temperature during summer is usually lower than 14-15 °C. The Secchi disk transparency is normally higher than 8-10 m in summer and autumn, but can be as low as 4.3 m during spring. The pH is about 6.0, and the specific conductivity about 1.0 mS m-1. The dominating cation and anion are Ca (≈0.8 mg 1-1 or 42 µeq 1-1), and SO4 (≈1.5 mg 1-1 or 33 µeq 1-1), respectively. The concentration of bicarbonate (HCO3, µeq 1-1) is sometimes lower, sometimes higher than that of SO4.