Natural convection with volumetric heating in a horizontal fluid layer with a rigid, insulated lower boundary and a rigid, isothermal upper boundary is experimentally investigated for Rayleigh numbers from 114 to 1. 8 multiplied by 10**6 times the critical value of linear stability theory. Joule heating by alternating current passing horizontally through the layer provides the volumetric energy source. Layer aspect ratios are kept small to minimize the effects of side walls. A correlation for mean Nusselt number is obtained for steady heat transfer, and data are presented on fluctuating temperatures at high Rayleigh numbers and on developing temperature distribution when the layer is subjected to a step change in power.