The effects of the Cretaceous-Tertiary (K/T) boundary global change on calcareous nannoplankton and planktic foraminifera are most severe in low latitudes and negligible in high latitudes. In low latitudes, species extinctions are complex and prolonged beginning during the final 100,000 to 300,000 yr of the Cretaceous, accelerating across the K/T boundary, and reaching maximum negative conditions between 10,000 and 40,000 yr into the Tertiary accompanied by low primary productivity. In high latitudes, no significant species extinctions occurred at or near the K/T boundary, and all dominant species thrived well into the early Tertiary. Return to a more stable ecosystem and to increased marine productivity in low latitudes does not occur until about 250,000 to 350,000 yr after the K/T boundary, coincident with the extinction of Cretaceous survivors in high latitudes. Within this transition interval, habitats of deep- and intermediate-dwelling tropical planktic foraminiferal species are gradually and selectively eliminated in low latitudes, and by K/T boundary time only cosmopolitan surface dwellers survive. This implies the disruption of the water-mass structure, change in the thermocline, and a drop in surface productivity. Although no single cause is likely to account for these different prolonged and dramatic faunal and environmental changes between low and high latitudes, long-term oceanic instability associated with sea-level, temperature, salinity, and productivity fluctuations may account for most of the faunal changes observed in planktic foraminifera. However, other environmental changes (e.g., volcanism, bolide impact) may have accelerated the demise of the low latitude Cretaceous fauna already on the decline.