Contact and non-contact atomic force microscopy (AFM) has been used for analyzing the influence of the defects in N-glycosidic process in mnn9 mutants of S. cerevisiae in the cell wall physical properties. High-resolution non-contact AFM image have shown the mutant cell surface to present large highly rough areas (compared with wild type ones) and also some irregular but compact structures usually associated to the former areas. Since no crater-like rings (scars) were observed on the surface of mutant cells (unlike high-resolution imaging of these surface features in wild type cells), these structures are suggested to be deformed scars. These results would also confirm a critical influence of mannoproteins in the zone of the septum, which was already suggested in previous works. Force curves obtained on the irregular and rough areas have shown them to be physically softer than other parts of the cell surface and than wild type cells, and were easily deformed by the AFM tip while scanning in the contact mode. These results could be taken as a direct verification of the known highly osmotic fragility of these mutants. This is at our knowledge the first time defects on cell wall on mnn9 mutants have been directly probed and observed at nanometer scale. (C) 2004 Elsevier B.V. All rights reserved.