An optical biosensor has been designed to quantify the amount of phenylalanine in new born babies. Rapid detection and quantification of phenylalanine in the serum has been achieved by immobilizing phenylalanine ammonia lyase in polyvinyl alcohol-jellose biocomposite films using entrapment method. The trans-cinnamic acid formed by the enzymatic reaction is measured using optical fibre spectrophotometry. The results obtained by this method are confirmed using HPLC and microassay methods. The optimum measurement of trans-cinnamic acid formation from phenylalanine is achieved in 150 mM Tris HCl buffer pH 8.5 at 30 degrees C and the lowest detectable concentration under these conditions is found to be 1.0 mu mol/L and response time obtained is 30 min. The biosensor developed is stable for atleast 1 month when stored in buffer at 4 degrees C and less than 10% drop in original activity is observed. The sensor developed requires minimum chemicals and the enzyme source used is also very cheap. Thus, the sensor has the credentials of commercial importance.