The onset of convection in a porous layer which is heated from below is considered. In particular we seek to determine the effect of spatially periodic variations in the permeability field on the identity of the onset mode as a function of both the period P of this variation and its amplitude A. A Floquet theory is assumed in order to ensure that the analysis is as general as possible. It is found that convection is always three-dimensional and that the critical Rayleigh number always decreases as either the period or the amplitude of the permeability variation increases. Furthermore, the corresponding Floquet exponent ν is either 0 or 1, and the range of values of P over which ν = 1 corresponds to the favoured mode has been obtained as a function of A.