A detailed quantitative study of well-preserved nannofossil assemblages in closely spaced samples across the Cretaceous-Tertiary (K-T) boundary at El Kef, Tunisia, reveals no extinctions of nannoplankton in the uppermost 4 m of Maastrichtian marls as has been reported for planktonic foraminifers from the same samples. Nannofossil assemblages below the boundary show no trends to indicate that ecologic stresses such as temperature change foreshadowed the K-T event in the Tethyan region. On the other hand, the disappearance of several species of the more depth-sensitive planktonic foraminifers below the boundary at El Kef is probably the result of sea-level regression (or sampling effects) rather than preboundary climate change or other environmental stresses. Cretaceous nannofossil specimens present above the K-T boundary at El Kef are interpreted to have been reworked, rather than having survived the K-T event. This interpretation is suggested by abundance patterns of common Cretaceous species and a correlation with short-term sea-level lowstands.