A new bottom-towed scintillometer has been deployed in the tropical Atlantic to investigate whether lithological changes on the deep-sea bed can be determined from variations in its natural radioactivity. Although count rates over the blanket of recent pelagic carbonates fall within the seawater background, high rates of gamma ray emission were detected on the crest of a seamount near 9¿N, 21¿W. Seismic profiles, bottom sampling and photography demonstrate that the radiometric anomalies coincide with areas in which the veneer of pelagic ooze is locally absent and phosphatized limestones of Cenozoic age (P2O5>14%) are exposed. The contrast in the U contents of the phosphorites (~30 ppm) and pelagic carbonates (100 ppm), profiling the levels of seabed radioactivity would be a relatively rapid means of defining the distribution of phosphorite in many parts of the oceans. ¿ American Geophysical Union 1989