A new optical fiber sensor for monitoring tetracycline has been described, based on the fluorescence quenching of 1,4-bis(5,5′-dimethylbenzoxazole-1′,3′-yl-2′)benzene incorporated into a thin plasticized polymer film by tetracycline extracted from aqueous phase into film phase. The sensor is fully reversible and highly reproducible. Furthermore, the sensor exhibits a linear response to tetracycline in the range 6.98×10 −7–8.73×10 −5 mol l −1 with a detection limit of 1.06×10 −7 mol l −1, and with the response time <30 s. The response is also selective to tetracycline, with some common pharmaceutical species, alkali and alkali-earth metal salts being highly discriminated, suggesting that the sensor can be used to monitor tetracycline in three pharmaceutical preparations. The recovery of tetracycline from commercial formulations is 95.3–98.3%.