It is demonstrated that the use of LIF spectroscopy of CS molecules provides sensitive measurements of plasma electric fields with high spatial resolution. The experiment was performed in a helium glow discharge which was produced in a discharge vessel applying a voltage to two gold-plated electrodes (60 mm in diameter, 40 mm spacing). The CS molecule admixture was generated by a rf discharge in a gas flow system outside the discharge vessel. The plasma parameters in the negative glow discharge were obtained by probe measurements. The electron density was about and the electron temperature was 0.2 eV on the discharge axis for , and helium gas pressure . The measured electric field strength was in the range of and its spatial distribution in the cathode fall region is in good agreement with cathode fall theory.