Atomic force microscopy (AFM) was used to image the surface topography of living Saccharomyces cerevisiae cells at high resolution and to monitor enzyme digestion of the cell wall in real time. Apart from the presence of bud scars, the surface of native cells imaged in aqueous solution was homogeneous and smooth. Topographic images of the surface were recorded to a lateral resolution of 2 nm without significant modification of the surface morphology. Successive images of single cells were collected at fixed time intervals following addition of protease and amyloglucosidase solutions. Protease caused a progressive increase of surface roughness. Large depressions surrounded by protruding edges, approximately 50 nm in height, were formed and attributed to the erosion of the mannoprotein outer layer. By contrast, no modification of the cell surface was noted upon addition of amyloglucosidase, which was consistent with the cell wall biochemical composition. These results indicate that AFM is a complementary tool to electron microscopy in that it allows the surface of living cells to be explored directly in real time.