This paper presents an immunosensing system to detect Escherichia coli and Salmonella based on electrochemical impedance spectroscopy at interdigitated electrode structures. Our results show the importance of good electrode design in the final detection limit. Four different structures have been fabricated and functionalized. Biotinylated polyclonal antibodies have been immobilized on neutravidin-coated chips, and BSA has been used to avoid nonspecific adsorption. The immunosensor may be said to be capacitive since it is that part of the impedance used to monitor the presence of bacteria in phosphate buffer solution samples. Detection limits around 10(4)-10(5) cells mL(-1) have been reached using chips featuring interdigitated structures of less than 10 microm wide and 1.5 mm long. In both cases, the detection limits of the corresponding ELISA assays, using the same antibodies, was 1 order of magnitude higher (10(5)-10(6) cells mL(-1)). The analysis time, including sensor preparation was less than 5 h.