Variations in the populations of blue-green algae epiphytic on Cladophora prolifera were studied quantitatively for a year in three harbours of contrasting water quality in the area of Calvi (Corsica). The development of blue-green algae was maximal in the summer and minimal in winter and spring, with some differences between the harbours. Density of filaments or colonies varied between 15 and 179 per mm. Increasing pollution seems to bring about a decline in diversity that especially adversely affect the large Lyngbya spp., the Spirulina spp. and the heterocystous taxa (Calothrix and Microchaete); however it leads to an increase in the abundance of some Phormidium spp. and Pleurocapsales. Those trends which had previously been qualitatively observed are thus confirmed quantitatively.