Mercury (Hg) is a toxic metal classified as a dangerous priority substance to be monitored in aquatic ecosystems by the European Water Framework Directive (WFD), due to its characteristics of bioaccumulation and biomagnification in food webs. An Environmental Quality Standard for biota (EQSbiota) has been defined for Hg in order to classify water bodies and was set at 0.02 mg Hg.kg-1 fresh weight. The application of such a standard requires defining one or more species of fish that is able to integrate the mercury contamination of the river in which they live. In French Guiana, we are faced with the difficulty of implementing this standard without downgrading all the aquatic ecosystems. Indeed, this region presents a great contamination of mercury due to the lithology of its soils and its history related to goldmining. The aim of this study was to help designing appropriate monitoring program of Hg in fish for the implementation of EQS biota in this particular context. A database was created based on the mercury concentrations analyzed in fish muscle during 8 research programs carried out since 1990. The database counts approximately 3000 fishes distributed over 50 creeks and 6000 fishes caught at about 200 stations located on the rivers of French Guiana; it includes also various information for each fish (Hg concentration in muscle, standard length, weight, family, species, trophic level, sample station location, anthropic or natural pressure for each station, etc.). The first results show that fish species in creeks are different from those present in rivers. After statistical and geographical data processing, 6 to 8 bioindicators species were proposed for creeks and 3 for rivers. The identification of reference sites, however, remains a very delicate task, due to the history associated with goldmining: just 7 potential reference sites for creeks and 4 for rivers. Results on Hg contamination of indicator fish species at reference and contaminated sites will be presented and discussed according to anthropogenic pressure. A first evaluation of reference concentrations is proposed.