A thermographimetric method which allows for a quick and accurate estimation of intra- and extracellular water of microbial cells is reviewed and improved. Knowledge of these fractions is important for physiological as well as for toxicological investigations. Results of the study indicate that besides the species, nutrient availability and growth conditions affect the intracellular water content. Intra- and extracellular water, dry matter, volume and density of a single cell of Arthrobacter sp. are calculated. There are indications that intracellular compartments of eukaryotes could also be investigated with this method.