Although the clay in some moderately fine and fine-textured Bt horizons of soils of arid and mediterranean climates of the southwestern United States is highly oriented, no distinct illuvial clay skins can be recognized. The distribution of clay skins is related to shrink-swell potentials. Clay skins are absent in horizons having a shrink-swell potential of more than 4% or a masepic or omnisepic plasmic fabric; they are present in equivalent horizons having low shrink-swell potentials and an insepic or mosepic plasmic fabric. The clay content, mineralogy, and moisture regime of a Bt horizon in turn largely determine its potential to shrink and swell and hence determine its plasmic fabric. Evidence that clay illuviation has indeed taken place in these finer textured Bt horizons is based on four pairs of geographically associated soils with horizons of clay accumulation. Bt horizons of the coarser textured members of pairs have clay skins and the finer textured members do not. The distribution of biotite pseudomorphs in some of these pairs parallels the distribution of clay skins, suggesting that oriented bodies of clay can be destroyed. Clay orientation in one of the horizons was reformed experimentally to show that the highly oriented soil fabrics do not acquire their orientation by illuviation of clay. The studies further indicate that bodies of oriented clay in medium and fine-textured B horizons have been erroneously described as clay skins.