This paper investigates the gravitactic bioconvection in rectangular enclosures. The governing equations are the continuity, the Navier-Stokes equations with the Boussinesq approximation and the diffusion equation for the motile micro-organisms. The control volume method is used to solve numerically the complete set of governing equations. The effects of bioconvection Peclet number from 0.1 to 10 and the aspect ratio from I to 5 are investigated on the onset of bioconvection. It was found that the bifurcation was suberitical in all cases. The critical Rayleigh number is decreased with increasing bioconvection Peclet number and with increasing aspect ratio. (c) 2007 Elsevier Ltd. All rights reserved.