A three-dimensional numerical study of the natural convection in a cubical cavity heated from below is reported at moderate Rayleigh numbers for three Prandtl numbers Pr = 0.71, 10 and 130. The six walls are considered rigid and immobile, with isothermal horizontal plates and adiabatic lateral walls. The Boussinesq approximation for the variation of physical properties is assumed. Seven different structures, four single roll-type, two four roll-type and a toroidal roll, and several flow transitions have been identified in the steady and laminar regime for Ra less than or equal to 6 x 10(4) and Pr less than or equal to 130. Both, the dynamic and heat transfer characteristics of these seven structures are discussed. The effects of slightly changing the aspect ratio or tilting the cavity on the stability of the different structures are also analyzed. There is general agreement between the predicted average Nusselt number and available correlations for Rayleigh-Benard convection in rectangular enclosures and between two horizontal plates.