The ultrastructure of ferrugineous oolites of the locality of Fontaine du Landard (Savoie, France) has been studied with the electronic microscope through thin sections obtained with the ultramicrotome. The problem of the porosity of these oolites has been solved through the examination of large numbers of micrographies. The porosity is due to pores elongated parallely to their cortical envelops, to fissures and to micro-organisms. The texture of each cortex consists of a succession of three differents zones. Goetite is the main mineral of the oolites. Finally some hypothesis are suggested concerning the evolution of the oolites and the minerals of which they are composed.