Within the composite fermion model, it is anticipated that a periodic variation of the density in a two-dimensional electron system at half filling produces a periodic fictitious magnetic field. In regions of smaller density fictitious flux quanta are missing to compensate the external magnetic field. The observation of magnetic commensurability minima in the dc-transport response furnishes evidence for this fundamental aspect of the composite fermion picture. In general, composite fermions in these modulated systems behave largely equivalent to electrons moving in a periodic magnetic field.