Coated quartz crystal microbalances were modified with a surface-imprinting process using whole yeast cells. These molded polymer and sol-gel surfaces show honeycomb-like structures as shown by atomic force microscopy. Reinclusion of cells allows a selective on-line monitoring of these microorganism concentrations in water over 5 orders of magnitude. The sensitivity to cells holds up in growth media up to 21 g/L. Even cell fragments can be detected in flowing conditions. The highly robust polymers on the sensor devices are suitable for biotechnological applications.