Behaviour of the clay fraction during desiccation in microaggregated Ferralsols from Brazil: role of microstructure and organic matter. The physical properties of the microaggregates of Ferralsols are still under discussion. Two sites were selected for study: in each site, one soil under a native savannah (Cerrado) and another one under pasture were studied. Samples were analysed in the laboratory by using scanning electron microscopy and mercury porosimetry and by measuring the water content at −0.1 and −1.5 MPa potential. Results showed the presence of microaggregates 50 to 300 μm in size and bigger ones 500 to 1 000 μm in size. The latter were often packed in dense volumes 2 to 20 mm in size, which were numerous in the top soil under pasture. These dense volumes are casts and would result from an imbalance in the faunal population as a consequence of deforestation. Results showed also a shrinkage of the clayey phase for drying and this shrinkage increased with the carbon content of the soil. These results showed that microaggregates are not rigid fabrics and the significant role of organic matter in physical properties of the soils studied.  2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS