Results of recent studies on the mechanism of cellulose biosynthesis in higher plants and in the bacterium Acetobacter xylinum are compared and contrasted. In higher plants, the synthesizing complex is thought to be mobile in the fluid-mosaic plasma membrane, whereas it is stationary in cells of A. xylinum. Similar patterns of sensitivity to inhibitors of cellulose synthesis as well as to changes in transmembrane electrical potential are shared by both plants and A. xylinum. In vivo tracer studies with both types of organisms support the concept that UDP-glucose is a precursor of cellulose. The stimulatory effect of polyethylene glycol (PEG) on glucan synthetases from both plants and A. xylinum may relate to stabilization by PEG of enzyme-factor associations.