A flow-through optosensor for phenylalanine and tyrosine is described in this paper. The sensor is developed in conjunction with a flow-injection analysis system and uses immobilized β-cyclodextrin as the sensing agent. The analytical performance characteristics of the proposed sensor for analysis of very low levels of phenylalanine and tyrosine were as follows: the detection limits for phenylalanine and tyrosine were 0.20 μg ml−1and 8.9 ng ml−1, respectively. The observed relative standard deviations were 1.03% for 50 μg ml−1of phenylalanine (n = 7) and 3.6% for 0.1 μg ml−1of tyrosine (n = 7), respectively.