The quantitative study and high resolution sampling of an essentially continuous and expanded Cretaceous/ Paleogene (K/P) boundary section in Tunisia allow us to test the model of extinction in planktic foraminifera. The extinction at the Aïn Settara section occurred over a short period of time similar to the Tunisian sections at El Kef and Elles and the Spanish sections at Agost, Caravaca and Zumaya. At Aïn Settara only 3 species disappeared in the latest Maastrichtian, 45 became extinct precisely at the K/P boundary and 18 disappeared in the earliest Danian. The species that became extinct at the K/P boundary constitute about 20% of the individuals in the population larger than 63 microns and 68% of the species, which suddenly became extinct in a catastrophic event precisely coinciding with the layer containing evidence for an asteroid impact. Most of these species are large, complex and low latitude deeper to intermediate dwelling forms. This extinction event is clearly the most important catastrophic mass extinction recorded in the history of planktic foraminifera. This pattern of extinction is superimposed on a controversial gradual pattern of extinction of 21 species that apparently began in the latest Maastrichtian and ended in the early Danian. The Maastrichtian species that seem to become extinct gradually are generally small, cosmopolitan and simple surface dwellers. The catastrophic mass extinction of the 45 species coincident with the K/P boundary is compatible with the effect of the impact of a large asteroid, whereas the gradual extinction of 18 species in the basal Danian could also be attributed to the long term disruptive effect of the impact.