The hygienic control of polluted surface waters has to be correlated with measurements of the decomposition rate of heterotrophic microbial populations. Starting in 1970 different methods have been tested for this purpose. Uptakekinetic studies of radioactive labelled substrates as well as automated BOD-registrations can not be handled as flow systems. Flow-Microcalorimetry (heat-conduction-principle) could be an easier approach to test the activity of heterotrophic aquatic populations. Probes from a trickling-filter outflow, from an oxidation pond and from a small river were tested simultaneously in a Flow-Microcalorimeter (LKB, 2107, Fig. 1) and a Drop-Microcalorimeter (WADSO, 1974 (fig. 2)) after adding 200 mug/L of Glucose. The resulting voltage/timecurves (Fig. 4,5,6) show a good correlation to the heterotrophic capacity of the probes. The minimal detectable continnous heat effect was 1 muW corresponding to a (relative) activity of 5.6-10(4) bacterial colonies/ml on Difco agar (counts after 48 hours incubation at 22 degrees C incubation temperature). A modification of the Flow-Microcalorimeter using a 5-10 times larger reaction vessel could enable the system to be used also in testing scarcely polluted surface waters and for toxicity tests of surface water samples.