Study of organic-walled phytoplankton (prasinophytes and “acritarchs”) from samples spanning the Ordovician/Silurian boundary in the southern Appalachians reveals an abrupt change in the composition of phytoplankton associations at the boundary, coincident with the extinction of several Ordovician species. The actual duration of the extinction is poorly constrained by available biostratigraphic evidence, but it apparently took place within the duration of one stage. Evidence from other regions suggests that the extinction was widespread and may have been worldwide. Within limits of resolution, the abrupt extinction among the phytoplankton was coincident with the extinction of benthic communities. Hypotheses calling upon species-area effects, evolutionary changes in the terrestrial flora, or gradual climatic deterioration to explain the terminal Ordovician extinction are not supported by evidence from the phytoplankton. A bolide impact model cannot be excluded with available evidence, nor can models calling upon threshold effects associated with changing climate.