The effect of aspect ratio (A) on heat transfer in differentially heated, air filled rectangular cavities with perfectly conducting horizontal walls is numerically studied. First the relationship between the vertical wall and vertical midline average Nusselt numbers is discussed, and the latter is found to be more representative of overall heat transfer with the present boundary condition. At each Rayleigh number, there is an aspect ratio for which a maximum in midline Nusselt number occurs. The maximum occurs at aspect ratios for which the thermally stratified zone is thin. Accordingly, the values of A that cause maxima in midline Nusselt number decrease as Ra grows. At each Ra, the average wall Nusselt number also has a maximum. Its value is lower and less notorious than the maximum in midline Nusselt number, and occurs at a higher aspect ratio. © 2013 Pushpa Publishing House.