The heat flow through the floors of five small lakes of known thermal history on the Canadian Shield was measured with a modified Bullard probe. A small correction for seasonal bottom water temperature variations was applied to temperature gradient measurements, and the heat flows are corrected for glaciation, lateral temperature gradients, sedimentation rates, and lateral thermal conductivity changes. Four lakes have an average heat flow of 49 ± 4 mW/m2 (1.2 ± 0.1 μcal/cm2 s). A high heat flow in the fifth lake is thought due to unusual refraction effects. The heat generation–heat flux combination yields a point that falls near accepted lines for the Canadian Shield.