In this paper, we reexamine the opportunistic polychaete/amphipod ratio, modifying it to allow estuarine and coastal communities to be divided into the five classes suggested by the European Water Framework Directive (WFD). The resulting biological index, called the BOPA index, considers the total number of individuals collected in the samples, the frequency of opportunistic polychaetes, and the frequency of amphipods (except the genus Jassa). After comparing this new index to AMBI and BENTIX, two other indices that have been proposed in the literature, we tested it in two situations involving soft-bottom communities in the English Channel (Bay of Morlaix and Bay of Seine). Our results show that the BOPA index is simple to use. Amphipods and opportunistic polychaetes (21 species, nine genus and two families from the AZTI list for a total of 3459 taxa) are easy to identify, providing that both the number of these organisms in a sample and the total number of individuals collected (independent of the sampling surface) is known. The BOPA is appropriate for use in the poorest communities whose total number of individuals exceeds 20 individuals.