Artificial fertilization depends on the oocyte quality, especially on the zona pellucida. This gelatinous outer layer becomes soft before and hardens after sperm penetration. Here, we propose a setup that characterizes the stiffness of the zona pellucida of mouse oocytes by electrical impedance spectroscopy. Single oocytes are hydrodynamically trapped at an aperture, which is located between two ring-shaped electrodes. By applying weak negative pressures to the cell trap, the electrical impedance correlates with the stiffness of the zona pellucida.