The paper deals with the relationship betweeen the water content of washed Serratia marcescens and the thermodynamic activity of water, a,, over the range 0 < a, < 1.0. Two overlapping types of measurement are used: osmotic water exchange and water-vapour sorption. The data are given in terms of the equation w = Bm-' where w is water content, m is external osmolality (actual or effective, depen...