Reference measurements were carried out using a universal biosensor based on a bacterial culture P. Fluorescens P75. The sensitivity of A. hydrophila biosensor to fatty substances was improved through adaptation of membranes in fat containing solution for 5–6 days in which biodegrading capabili- ties of microorganisms improved and for low fat concentrations the discrepancy between the biosensor measured BOD and BOD7 was less than 15%. A semi-specific biosensor enabled to measure BOD in high fat content wastewaters, both synthetic and industrial, more accurately than a universal biosensor.