Dinoflagellate cysts and planktonic foraminifers have been studied from the Cretaceous/Tertiary (K/T) boundary interval at El Haria (northwest Tunisia). A high-resolution integrated biostratigraphy is presented. The K/T boundary is drawn at the level of extinction of Cretaceous planktonic foraminifers and is coincident with the first occurrence of the dinoflagellate cyst species Danea californica. The final extinction of planktonic foraminifers is foreshadowed by a reduction in their total abundance some 5 kyr earlier at the base of the boundary clay. This reduction is coeval with reported anomalies in siderophyle elements and δ13C-values in the same area. Dinoflagellate cysts do not show accelerated rates of extinction at K/T time. Associations of dinoflagellate cysts, however, change drastically and parallel changes in relative numbers of sporomorphs (spores and pollen) and in the quantity of land-derived organic matter. Jointly, these changes reflect a rapidly falling sea level during the final 17 kyr of the Mesozoic which culminates at the level of the K/T boundary. This steep sea level fall at K/T time represents a peak regressive pulse at the end of the well-documented latest Cretaceous regressive trend. This short-term sea level fall might show to be a wide-spread phenomenon which could have caused an excess shrinking of the already reduced areal extent of marginal seas. Since deep waters in Cretaceous oceans were primarily produced in shallow marginal seas, the rate of formation of deep water might have been minimized at K/T time. Minimum rates of formation of deep water might have curtailed the slow upward mixing of relatively cool and nutrient-rich deeper water through which the thermocline weakened and surficial waters became depleted in nutrients. Consequently, phytoplankton productivity rapidly diminished which, in combination with a weakened thermal gradient, pushed the highly depth-stratified Cretaceous planktonic foraminiferal fauna to extinction over a period of time of some 5 kyr. Guembelitria cretacea was the sole planktonic foraminifer which could accommodate to the low productivity conditions. The oscillating rise in sea level at the beginning of the Cenozoic reinforced the upward mixing of relatively cool and nutrient-rich deeper water, steepened the thermocline and replenished the photic layer with nutrients. Concomitant niche-differentiation in the photic layer progressively stimulated morphological innovation amongst early Cenozoic planktonic foraminifers. The final return of normally-sized planktonic foraminifers and of stable and well-balanced dinoflagellate cyst associations at about 125 kyr after the K/T boundary seems to indicate that primary productivity and niche differentiation in the photic layer begin to revert to optimum levels. The earliest Cenozoic planktonic foraminiferal species Globoconusa minutula and Parvularugoglobigerina fringa are thought to have developed from a benthic foraminiferal species rather than having a planktonic ancestry.