The biological autonomy of man when isolated from his original biosphere can be ensured only by Biological Life Support technologies. Among these, bioreactors are of prime importance and can be divided into two main groups: biomass producing bioreactors and materials transforming bioreactors. The latter case requires, if possible, high concentrations of active cells, in order to obtain a maximum efficiency in the transformation of substracts to products. One way to obtain such objectives is to immobilise the microorganisms. Specific and precise tools are required to control continuously the immobilised biomass. Among them there is a biomass monitor able to measure both free and immobilised biomass concentration.Such a monitor has been developed by Nuevas Tecnologías Espaciales (NTE) in collaboration with the Chemical Engineering Unit of the Universitat Autònoma de Barcelona (UAB) under the Technology Research Programme (TRP) of the European Space Agency (ESA). The method used is the measurement of the electrical impedance of a culture at different frequencies which should allow us to measure both free and immobilised cell concentration. The electrical impedance of biological systems decreases with the frequency. This decrease is proportional to the quantity of cells in the culture and the value of the frequency at which the decrease appears depends on the type of cells in the culture. A specific data processing software has been designed during this study.With this monitor, we have been able to measure biomass concentration in free and immobilised (entrapment in alginate beads) yeasts (Candida rugosa), free and immobilised (adsorption on microcarriers) bacteria (Rhodobacter capsulata). The minimum concentration detected has been 0.3 gr DW/1 for yeasts and 0.5 gr DW/1 for bacteria. Adaptation of this method to other types of cells is envisioned.