The French microbiological monitoring network (REMI) has been carried out since 1989 in order to assess faecal contamination levels (Escherichia coli) of shellfish production areas. It has been shown that species growing at the same site may show differences in microbiological contamination. The aim of this study is to investigate these differences and to examine how it can be used in the context of monitoring networks, notably in identifying indicator species. We would expect an indicator species to yield results at least as high as those of the other species present. Of the 372 monitoring points defined in the REMI in 2011, 20 points are sampled at the same time for at least two different species of shellfish, usually on a monthly basis. This study was organised in three stages : - identification of the results obtained in parallel at the same point, and the same time of sampling for at least two species. All data stored in the national data bank Quadrige² since 1989 have been extracted. - data treatment : our primary parameter is the logarithm of E. Coli concentration. Descriptive statistics completed with a non parametric tests have been performed. The comparison was done per couple of shellfish : Mytilus edulis / Cerastoderma edule, Mytilus galloprovincialis / Cerastoderma edule, Crassostrea gigas / Cerastoderma edule. - furthermore, as the classification in France is done per group of shellfish : Group 1 : gastropod, echinoderm, group 2 : filter-feeding burrowing bivalves (cockles, clams…), Group 3 : non burrowing filter-feeding bivalves (oysters, mussels), data treatment consisted in comparison of the difference between the groups. This data treatment confirmed that burrowing shellfish were significantly more contaminated than non burrowing bivalves and pointed out differences between species that will be more detailed in the presentation.