Destruction of all life in a single geographic region would lead to the “worldwide” extinction of those species confined to the affected region. Could purely regional biotic crises explain the mass extinctions seen in the fossil record? This proposition has been tested as follows: the geographic distributions of all families of living terrestrial vertebrates and of all genera of two marine groups (corals and echinoids) were digitized and stored on tape; target points were selected at random on the earth’s surface and several lethal radii were specified; for each target point and each lethal radius, a census was made of those taxa that would become extinct by virtue of their living only within the lethal area. The results show that modern biogeography is too robust for mass extinction to result from annihilation of life in a single region. Lethal areas exceeding half the earth’s surface are required before extinction reaches the levels typical for the mass extinctions of the Phanerozoic. Therefore, a global or near global crisis or environmental deterioration is required.