An optical fibre sensor is presented for the continuous measurement of fluorescent emissions from a fluid sample. The sensor employs a novel patented waveguiding system to allow separation of the excitation and emission wavelengths without optical filtering, has a working range of eight decades with fluorescein and is both chemically and physically robust. The device has applications in process control, flow tracing and environmental monitoring. The authors have developed a novel biochemical toxicity assay technique that produces a fluorescent indication of overall toxicity levels and this assay can be interfaced to the optical fibre sensor to form a rapid online toxicity monitoring device. The assay can be applied to any environment as the test organism is variable. Details of the sensor and aspects of the optical theory are presented together with the results of initial laboratory tests both of the sensor device and the toxicity assay technique.