This paper presents the fabrication and testing of a novel microelectromechanical (MEMS) biosensor based on live cells. The biosensor combines two biosensing techniques; resonant frequency measurements and electric cellsubstrate impedance sensing (ECIS) on a single device. The sensor is based on the innovative placement of the working microelectrode for ECIS technique as the upper electrode of a quartz crystal microbalance (QCM) resonator. This hybrid biosensor was tested with bovine aortic endothelial cells with different seeding densities. The cell attachment and spreading was monitored with both sensors; the QCM and the ECIS technique. After the cells form a monolayer the values of the impedance and resonant frequency measurements are constant. The optimal cell seeding density with minimal time required to attach and form a monolayer was observed to be 1.5×104 cells/cm2. This biosensor monitors the cells attachment and viability and could be used for screening toxicants in drinking water.