In the Ager basin and Benabarre area (southern Pyrenees, Spain), the Cretaceous-Tertiary (K/T) boundary is recorded in a succession of non-marine to coastal deposits (Lower Tremp Formation). Abundant diverse dinosaur tracks have been found at two localities on top of an estuarine sandstone body. A few meters above, grey marls at two localities contain Palaeocene mammals and fish. Both levels are correlated with the latest part of chron C29R. These data have led to the location of the K/T boundary within a 3 m thick stratigraphic interval with no major breaks in the sedimentation. A rapid fall in delta 13 C content is recorded shortly after the K/T boundary. The delta 13 C curve does not support the hypothesis of a gradual shift due to volcanic activity. Both fossil vertebrates and isotope data are compatible with an abrupt change in the continental ecosystems close to the K/T boundary.