The composition and structure of membrane biofouling in water treatment can be studied using specific stains (fluorochromes) and confocal laser scanning microscopy (CLSM). An image analysis program (CMem) has been developed to quantify parameters describing the biofilm structure on curved membrane surfaces, such as hollow fiber membranes. Two CLSM images stacks were acquired: one of the stained biofilm and one of the membrane underneath it. The curved reference surface in a three-dimensional stack of CLSM images was located, based upon detection of membrane autofluorescence/reflection signals. The stained components identified in the biofilm stack were then related to the membrane surface. Structural parameters such as thickness, volume of biomass, porosity and roughness of biofilms on curved membrane surfaces were computed.