The abrupt, widespread, and simultaneous extinction of radiolarian species makes them ideal stratigraphic markers. Five species became extinct in the North Pacific during the last 3 m.y. Four of these are used to define four stratigraphic zones. The boundary between the oldest two zones correlates with the Pliocene/Pleistocene boundary, as defined in southern Italy. These zones can be related through paleomagnetic stratigraphy to previously established radiolarian and foraminiferal zonations. One species (Eucyrtidium matuyamai) evolved and became extinct during the last 2.5 m.y. Its evolution can be related to the invasion of and adaptation to a new habitat. The extinction of E. matuyamai shows a striking correlation with the magnetic reversal at the base of the Jaramillo event. The rapid evolution of this species probably reduced the genetic variability of the population, making it more vulnerable to extinction than other less rapidly evolving species. The environmental change that caused the extinction is unclear; however, there is suggestive evidence that it is in some way related to a reversal of the earth’s field. If reversals cause abrupt environmental changes, they may have played an important selective role through geologic time.