The Guiana Shield is located in North-Eastern South America. The river system of the Atlantic slope of the Shield is composed of non-interconnected rivers. Although some connections exist, its freshwater ichtyofauna is in great part different from the Amazon basin one. In this study, we investigated the faunistic assemblage of the Guiana shield's river system. We redefined the map of the freshwater biogeographic areas of the northern Guiana Shield. We first excluded the Essequibo from the Guyana Shield system and assiocated it with the Amazonian fauna. In the Guyanas, we observed 3 regions: one in the west (Corantijn to Suriname), one in the East (Sinnamary to Oyapock) and one in the centre (Commewijne to Mana). In a second stage, we reconstructed the process of dispersion through these biogeographical areas. We studied genera of the Heptapteridae (Rhamdia, Chasmocranus and Pimelodella) using molecular data. We observed that these genera dispersed differently within the system: results indicated that Rhamdia and Pimelodella could overcome natural barriers and spread easier while Chasmocranus seemed more restricted.