The technology of flow cytometry and cell sorting has existed for ∼20 years, and its potential applications to oceanography have been obvious to many for nearly as long. Its introduction into oceanography did not occur, however, until the early 1980s [Yentsch et al. 1983; Olson et al. 1983]. The introduction was made possible largely t hrough the funding of instruments dedicated to oceanographic applications by the U.S. National Science Foundation, the Office of Naval Research, and private institutions. In the last 5 years, interest in this new tool has grown significantly, and the number of flow cytometry facilities dedicated to applications in oceanography and limnology in the United States, Canada, and Europe has expanded to a surprising degree (Table 1).