NOVELTY - A BOD-measuring biosensor (B1) comprises a measuring unit, an electric current detecting unit and recording unit for recording electric current. The measuring unit comprises a mediator-less biofuel cell involving: cathodic and anodic compartments containing cathode and anode respectively, in conductive medium and ion exchange membrane between the compartments. The anodic compartment is fed with a sample containing electrochemically active bacteria.    USE - For measuring biological oxygen demand (BOD) of waste water and sludge, measuring concentration of an organic substance; and for densely culturing electrochemically active bacteria present in waste water and sludge (all claimed).    ADVANTAGE - The biosensor enables the performance of simple and rapid measurement and is relatively inexpensive in cost of fabrication, application, maintenance and repair. The biosensor can be operated without an artificial addition of microorganisms, and allows the microorganism to be maintained as a suitable activity depending on nature of waste water. The biofuel cell used in the biosensor can be operated in a stable manner over more than six months. The biosensor employs more than one microorganism present in the waste water capable of surviving on complex nutrient components and hence give correct value of biological oxygen demand, unlike biocell containing only one microorganism species. The biosensor does not involve expensive replaceable porous membrane containing immobilize microorganisms or oxygen electrode. The biocell is electron mediator less or without a separate transducer as it employs electrochemically active bacteria capable of transferring electrons directly to outside.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) measuring biological oxygen demand of a sample using (B1) involves:    (a) electrically connecting the anode to the cathode via a resistor;    (b) introducing nitrogen into the anodic compartment to maintain an anaerobic condition and introducing oxygen or air into the cathodic compartment to maintain an aerobic condition;    (c) densely culturing an electrochemically active bacterium present in the sample in the anodic compartment; and    (d) measuring electric current generated by employing the densely cultured, electrochemically active bacteria as a microbial catalyst;    (2) a mediator-less biofuel cell type biosensor (B2) for measuring the organic substance concentration comprising the measuring unit comprising the mediator-less biofuel cell, an electric current detecting unit and the recording unit. The anodic compartment of the biofuel cell contains a sample containing a single species of an electrochemically active bacterium to catabolize the desired organic substance;    (3) measuring the concentration of an organic substance using (B2) involves:    (a) adding a sample to be measured to the anodic compartment while continuing to feed air to the cathodic compartment to maintain the cathodic compartment at a voltage different from the anodic compartment; and    (b) measuring an electric current generated from the consumption of the organic substance contained in the sample by the electrochemically active bacterium, to measure the concentration of the organic substance; and    (4) densely culturing electrochemically active bacteria present in active sludge and wastewater using (B2) involves:    (a) adding the active sludge and the wastewater to the anodic compartment;    (b) electrically connecting the anodic compartment to the cathodic compartment via a resistor; and    (c) introducing nitrogen to the anodic compartment to maintain the compartment in an anaerobic condition, while introducing air to the cathodic compartment to maintain the compartment in an aerobic condition such that the bacterium present in the active sludge and waste water is densely cultured without a separate electron receptor.