The extinction of planktonic foraminiferal species across the Cretaceous/Tertiary (K/T) boundary has been examined in continental-shelf sections at El Kef, Tunisia, and Brazos River, Texas. These sections are considered to contain the most complete boundary transition record known to date. In both sections, an extended period of species extinctions spans from about 300,000 yr below to about 200,000-300,000 yr above the K/T boundary. Distinct episodes of accelerated extinctions occur below the boundary and about 50,000 yr above the boundary. At El Kef, only 26% of the species extinctions appear directly associated with the K/T boundary and iridium anomaly. At Brazos River, no species extinctions or measurable faunal changes appear directly associated with the K/T boundary and iridium anomaly. Species extinctions selectively affect large, ornate, tropical to subtropical species first and small, primitive, nonornate, subtropical to temperate species last. This pattern of species extinction is likely caused by increased ecological stresses as a result of a late Maastrichtian sea-level regression and global cooling. The extended period of species extinctions and absence of extinctions at the K/T boundary at Brazos River is not entirely compatible with either impact or volcanism theories. Perhaps, multiple unrelated causes should be considered, including a sea-level regression, global cooling, a K/T boundary impact of limited extent, and extensive volcanism.