A full 3-D numerical study is reported on natural convection of air in six heated-from-below parallelepipedic enclosures. A supercritical Rayleigh number (Ra = 8 x 10(3)), was used in order to track the evolution of convective structures as a function of the enclosure’s aspect ratio (A) only. The six situations (1 less-than-or-equal-to A less-than-or-equal-to 5) exhibit a characteristic toroidal flow pattern which can be seen, at the lowest aspect ratio, as a unicellular structure which evolves to a multicellular combination of concentric roll-cells at higher aspect ratios. The transition from one convective structure to another has the features of a flow bifurcation controlled by A. The overall Nusselt number changed continuously when the aspect ratio was increased. At the lowest aspect ratios the change was significant, but further increments in A did not produce important variations in this parameter, in the range of aspect ratios studied.