A potential hand-held biosensor system for the in-situ analysis of toxic algae was developed during the EU-project ALGADEC. Identification of toxic algae is based on molecular probes that specifically target its rRNA. 17 taxon specific probe sets were developed for harmful algae that occur in three different coastal areas in Europe. A sandwich-hybridization and two labelled probes are used to detect the rRNA. A capture probe, immobilised on the biosensor, binds to RNA-strands isolated from the target organism. A second digoxigen-labelled probe binds also to the RNA-strands. After incubation with an antibody-enzyme complex directed against digoxigenin, a substrate is added and a redox-reaction takes place. The resulting electrical current is measured and the amount of bound rRNA is proportional to the electrical current. The adaptation to the sensor and the probe specificity tests were done using laboratory strains with closely related species to avoid false positives and to guarantee that only desired strains are detected. The signals from the different probes are recorded by a microcontroller unit. If a PC is connected to the system, an easy to operate software visualizes process data, graphic results, and the measured values will be stored on the hard disc. The main steps of the analysis process are executed automatically in the measurement device. Only a manual filtering, including a lysis procedure has to be done before the automatic measurement. The portable ALGADEC device is also capable to operate as a stand-alone system with A bloom is regarded as a sudden increase in the microalgal population activated by suitable growth conditions so that concentrations of 104-105 build in keypad, display, power supply and memory card. In a new project, the ALGADEC device shall be further automated, optimized and implemented into a FerryBox system. © Springer Science + Business Media B.V. 2008.