Somatic coliphages, F-specific phages and phages infecting Bacteriodes fragilis were determined in: sea water at variable distances from the freshwater polluting foci, in mussels collected at sites with different levels of faecal pollution of overlaying waters and in mussels purchased in markets. The relative proportions of the different bacteriophages are similar to those in polluting freshwater both in waters and mussels collected in areas near the sites where polluted freshwater reaches the sea. However, as we move further from the origin of pollution there is a very significant increase in the relative proportion of somatic coliphages and particularly of phages infecting B. fragilis.