A single-use phosphorimetric sensor to determine the germicide nalidixic acid is proposed. The sensing action is based on the absorption of the analyte into the sensing zone and the subsequent measurement of the phosphorescence intensity emitted by the analyte fixed in the sensor. This plane drop sensor is made up of a 3 x 1.6 cm sheet of the polyester Mylar as solid support, and a circular film 5 mm in diameter and 20 microns in thickness, formed by poly(vinyl chloride) and tributyl phosphate as the plasticizer, adhered to its surface. The sensor is introduced for 2 h into the sample solution, after which it is dried and the phosphorescence intensity is measured directly at lambda ex = 332 nm, lambda em = 412 nm, with a delay time of 0.15 ms and a gate time of 10 ms, under a dry nitrogen stream. The characteristic parameters of the construction of the sensing zone and of the processes of fixing the analyte along with the emission of phosphorescence were studied. The applicable concentration range was from 60 to 1500 ng ml-1, with a detection limit of 20 ng ml-1 and a precision of 2% expressed as relative standard deviation. The method was applied to the determination of nalidixic acid in milk and human urine with recoveries ranging between 96.0 and 103.7%. The calibration process was carried out by applying a mathematical method of finite elements that expresses the analytical signal as a function of the analyte concentration and equilibration time between the sensor and the sample solution.