In addition to the conventional hydrological research carried out by IDESP in the region of the Marajó Islands, a study has been undertaken of the natural variations of hydrogen and oxigen isotopes in water. Measurements have been taken of D and 18O concentrations in water from precipitation, rivers lakes inland of Marajó Island and surface waters around it. The data obtained are discussed as to the applicability of these techniques to hydrological studies in the Marajó region. The difference in the isotopic composition between the waters from the Amazon River near its north edge and those from the Marajó bay to the south appears distinctly, in spite of the big volume of water from the Amazon discharged into the Marajó bay, as detected through isotopic analyses of waters from the "Furo" of Breves (1). The internal surface waters in the Island present isotopic composition that is slightly different from that of rain waters the former ones being more enriched in heavy isotopes.