A white light optical fibre fluorosensor for water quality measurements of the phytoplankton population is reported. The excitation light is amplitude modulated and a phase sensitive detection system is employed. The spectral properties of two typical samples of phytoplankton obtained off the coast of Scotland have been characterised and used in laboratory tests of the fluorosensor. Initial results show that this system is capable of resolving fluorescein concentrations in water of 10−8mol 1−1and phytoplankton populations of 0.1 μgl.