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Little has been known about the earliest Toba eruptive episodes that created the largest-known caldera complex of Quaternary age. Here we report evidence for the eastward dispersal of the oldest Toba tuff in South China Sea sediments to 2500 km away from the source. The tephra deposits occur below the Brunhes-Matuyama geomagnetic boundary (778 ka) and slightly above the Australasian microtektite layer (793 ka). Calibrated by astronomically tuned oxygen isotope stratigraphy, the middle Pleistocene Toba eruption occurred during the deglaciation at 788 [+ or -] 2.2 ka, according to the tephra occurrence between marine isotope stages 20 and 19. This refined age is in good agreement with the [sup.40]Ar/[sup.39]Ar date of 800 [+ or -] 20 ka for the Toba tephra (layer D) from Ocean Drilling Program (ODP) Site 758, but significantly younger than the commonly cited Ar/Ar age of 840 [+ or -] 30 ka. The eruption expelled at least 800-1000 [km.sup.3] dense-rock-equivalent of rhyolitic magma on the basis of the widespread tephra-fall deposit in the basins of the Indian Ocean and the South China Sea. In spite of its exceptional magnitude, the timing of this major eruption does not indicate a causal linkage between this event and a long-term global climatic deterioration. Keywords: Toba tuffs, microtektites, tephrochronology, explosive eruptions, climate effects.