A fiber-optic biosensor for glucose concentration detection has been designed, based on electrostatic self-assembly. The end face of a multimode fiber is coated with poly(allylamine hydrochloride) in combination with Prussian blue and the enzyme glucose oxidase. The glucose concentration that can be measured ranges between 0.06 and 2 mM. The initial velocity of change of the output biosensor signal has been found to be linear in the glucose concentration. The sensor recovers after immersion in ascorbic acid. The pH values analyzed range between 5 and 7. The sensitivity of the device has been increased by adjusting the number of bilayers and by adding some capping bilayers at the end of the structure.