The water chemistry, flora, and fauna of Lake Rotokawa (38° 37.8’ S, 176° 11.2'E) was studied in 1975–76. The mean pH is 2.1 and thermal inflows may elevate the mean summer temperature of the surface waters 4.2°c above that of nearby cold water Lake Rotongaio (18.9°c). The temperature range of surface water was from 10.1 °c in winter to 23.1°c in summer. The major anions were SO42−679 g.m−3, and Cl‐ 314 g.m−3. Mean concentrations of major cations were Na+224 g.m−3, K+28.9 g.m−3, Ca2+13.3 g.m−3, and Mg2+2.6 g.m−3.