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The knowledge of the active biomass amount and its characterisation is of primary importance for the management and for the design of wastewater treatment plants on the basis of the recently developed models. OUR curves obtained in aerobic exponential growth tests are proposed by several authors as an indirect method to estimate the active fraction. The aim of this work is the application of a direct method to measure the viable biomass based on flow cytometry techniques and the comparison with the active fraction obtained from respirometric tests. To assess the viable fraction of a biomass expressed in terms of COD concentration it is necessary to estimate the biovolume of viable bacteria and to know the specific carbon content per cell. For the investigated activated sludge samples, the bacterial biomass measured by FCM was 588 mgCOD L(-1) on average in a two-months period. This value of active fraction corresponds to 14% of particulate COD. Active biomass values measured with the respirometric approach were consistent but generally higher than those obtained by FCM.