This article describes the preparation and performance of an optical sensor for continuous measurement of chloride at extracellular (serum) levels (20-200 mM). The sensor is based on dynamic quenching of the fluorescence of lucigenin which was photo-immobilized on a hydrogel. Quenching occurs via a collisional mechanism, and the decrease in fluorescence intensity on exposure to 100 mM chloride typically is -60%. It allows the determination of chloride in the 1 to 200 mM concentration range, with a precision of +/-3 mM at 120 mM. Bromide, iodide and salicylate act as interferents, while the effect of pH and oxygen is negligibly small. The sensor displays strong fluorescence intensity, excellent reproducibility, long-term stability, response times in the order of 2-5 min, and is used in a commercial serum and blood chloride analyzer.