This paper investigates the effect of heating or cooling from below on the development of gravitactic bioconvection in a square enclosure with stress free sidewalls. The governing equations are the Navier-Stokes equations with the Boussinesq approximation, the diffusion equation for the motile microorganism and the energy equation for the temperature. The control volume method is used to solve numerically the complete set of governing equations. It was found that the suspension is destabilized by heating from below and stabilized by cooling from below. A transition from a subcritical bifurcation to a supercritical bifurcation was observed in the case of heating from below when the thermal Rayleigh number was increased. (c) 2006 Elsevier Ltd. All rights reserved.