A conductometric proteinase K biosensor for organic matter monitoring in rivers has been developed. In fact, with approximately 30% of the total Chemical Oxygen Demand (COD), proteins were chosen to be used as indicators of urban pollution. Proteinase K hydrolyzes proteins into different ionic amino acids which results in local conductivity changes. In this work, we began with the optimization of biosensor response using bovine serum albumin (BSA) as standard protein. A stable biosensor with a constant repeatability and a detection limit about 0.5 mg/ml BSA were obtained. Then, response biosensor was tested with samples of rivers water. Good correlations between conductance changes and values given by standard methods (chemical oxygen demand and protein concentration evaluated by microBCA protein assay) have been shown.