Unstably stratified turbulent channel flow with walls kept at constant but different temperatures is studied using large-eddy simulation (LES). The low-Mach number approximation is applied to the Navier-Stokes equations. In contrast to the Boussinesq approximation, which is limited to small values of density variations, the low-Mach number approximation allows higher values of density variations. The variations of density, as well as other fluid properties, such as the dynamic viscosity, and the thermal conductivity are taken into account. Two test cases with high and low wall temperature, or density, ratios of 1.01 and 4 are considered. For both cases different Richardson numbers are examined. A systematic investigation of the flow regimes is reported. The mean flow properties and fluctuating values are presented. Two cases, a stratified case with a density ratio of 1.01, and an unstratified case with a density ratio of 2, are validated against the available data in the literature. (C) 2010 Elsevier Ltd. All rights reserved.