Structural studies in the ultramafic transition zone from eight ophiolite complexes bring new data assembled in cross sections. They illustrate the remarkable similarities between transition zones in the various massifs, the most noticeable dissimilarities being in the thickness of these zones (10 m to over 1000 m) and probably also in the original attitude of the formations. Otherwise they have in common (1) a lower limit of dunite above harzburgite which is altogether compositionally gradational and irregular in the field: a residual origin is ascribed to these basal dunites; (2) a dominantly dunitic composition with chromite seams and pods which are also present in the underlying harzburgites but, up section, an enrichment in clinopyroxene and feldspar creating wehrlites and troctolites; (3) a clear-cut contact with the overlying gabbroic cumulates. This petrological limit tends to coincide with the upper limit of high T plastic flow foliation. Very large strain is recorded just beneath this contact.