An optical fibre sensor has been developed to measure particle concentration in water. The principle of operation is based on light being coupled between two parallel mounted fibres in the vicinity of the sensing region. The optical power is coupled by means of the evanescent field of the multimode fibres. A theoretical description of the light propagation mechanism in the fibre is presented which is extended to include the effect of attenuation of the evanescent wave in the measurand medium. Experimental results are also presented for yeast suspensions in water in the range 0-16 gl-1. \circledC 2000 The Institute of Measurement and Control.