Measurements of heat flow at 28 sites in the bottom sediments of Lake Rotomahana indicate a total conductive beat output of 19 MW, and an average conductive heat flux three times higher than in the thermal areas beneath Lakes Rotorua and Taupo. High heat flows (averaging 4 W/m(2)) occur in the south-west part of the lake over an area of 4.8 km(2), representing just over half the area of the lake. Very high heat flows (greater than 10 W/m(2) and up to 48 W/m(2)) occur in two parts of me high heat Bow anomaly: one (of 1 km(2)) in the west which extends onto an active part of the Waimangu thermal region on the land, and the other (of 0.5 km(2)) in the southern part of the lake which appeared to be a separate entity. In the north-eastern remainder of the lake no high heat flows were measured.