Comparison of planktic foraminiferal assemblages from sections at Koshak and Kyzylsai (boreal Paratethys) allows an analysis of the biotic crisis occurred during the Cretaceous-Tertiary transition (K-T) in the Mangyslak Peninsula. Foraminiferal assemblages in both sections are typically oligotaxic, with low species richness and a single dominant species (Chiloguembelina waiparaensis), through the late Maastrchtian and early Danian. In the latest Maastrichtian in the Koshak and Kyzylsai sections, a group of exotic planktic foraminifera from low latitudes appears in the assamblage, suggesting seawater warming and a rising sea level. These are tropical-subtropical, large and ornate planktic foraminifera that account for less than 25% of the assemblage and they disappear before or at the K/T boundary in both sections. The species that survive the K/T boundary are the endemic, small, cosmopolitan opportunists, they are biserial, triserial and trochospiral taxa of simple morphologies, mostly surface dwellers or able to tolerate low oxygen conditions. These survivor taxa account for more than 75% of the assemblage. Decreased primary productivity and oxygen during the latest Maastrichtian are suggested as the main factors for the K/T biotic crisis and its ecologic selectivity in the low latitudes. However, in high latitudes environmental conditions (including productivity) changed little and this resulted in a less catastrophic K/T boundary biotic event.