Two holes were drilled to depths of about 180 m in the Lower Klamath Lake basin south of Klamath Falls, Oregon, to obtain heat flow data and to provide estimates of the thermal conductivity of the valley fill. Twenty-nine thermal conductivity determinations on eight cores give a mean conductivity of 1.82 meal/cm s ¿C (0.75 W/m ¿K). Curvature in the upper 50 m of both temperature profiles indicates a decrease in surface temperature of about 1.8¿C, presumably resulting from reclamation of what was marshland in the early part of this century. A surprisingly low heat flow of 0.3 HFU (F HFU=10-6 cal/cm2s=41.8 mW/cm2) was measured at site LS near the center of the basin. At site OC-1,7 km east of LS and 2 km from the Klamath Hills geothermal zone, the heat flow was 1.44 HFU, also a low value in this setting. Temperature profiles in 15 unused water wells in the area had linear gradients ranging from 47¿ to 170¿C/km. The corresponding lower limits of heat flow (conductivities measured at the two heat flow sites being used) range from 0.8 to 3.1 HFU. These variations in heat flow evidently are caused by temperature variations in a convecting system within the near-surface volcanic rocks and do not provide firm constraints on the nature sources at depth.