An all-solid-state green fluorescent protein (GFP) sensor for GFP measurement was developed. It is immune to interference from ambient light and works with standard flow-through cuvettes. The sensor is practically insensitive to the scattered excitation light encountered in microbial suspensions. It has a range of 0.0002-1 g/L (7.4 x 10(-9) - 3.7 x 10(-5) M) with limit of detection 0.00019 g/L (7.0 x 10(-9) M). The sensor could be used with a UV or blue light emitting diode (LED) as a light source, depending on required sensitivity, selectivity, and background levels. Its very low cost makes it useful in a variety of applications. This article describes the construction and validation of the sensor both off- and on-line in fermentation processes.