High resolution (~5-10 kyr) planktonic foraminiferal analysis at Elles, Tunisia, reveals major changes in the structure of the Tethyan marine ecosystem during the upper Maastrichtian. During the first 1.5 Myr of the late Maastrichtian (68.3-66.8 Ma) relatively stable environmental conditions and cool temperatures are indicated by diverse planktonic foraminiferal populations with abundant intermediate and surface dwellers. A progressive cooling trend between ~66.8-65.45 Ma resulted in the decline of globotruncanid species (intermediate dwellers). This group experienced a further decline at the climax of a rapid warm event about 300 kyr before the K-T boundary. At the same time relative abundances of long ranging dominant species fluctuated considerably reflecting the high stress environmental conditions. Times of critical high stress environments during the late Maastrichtian, and particularly at the K-T boundary, are indicated by low species diversity and blooms of the opportunistic genus Guembelitria at warm-cool transition intervals. During the last 100 kyr of the Maastrichtian rapid cooling is associated with accelerated species extinctions followed by the extinction of all tropical and subtropical species at the K-T boundary.  PDF