Flow cytometry has been used to study phytoplankton since the early 1980s. Because of the wide range of cell sizes and concentrations that occur in natural samples, multiple instrument configurations have been required to characterize pico, ultra-, and nanophytoplankton. Lengthy changeover times between configurations have made synoptic analyses of phytoplankton communities extremely cumbersome and therefore rare. To overcome this problem, we have added dual-sheath capability and adjustable optics to a commercially available flow cytometer. These additions expand the instrument's capabilities during routine operation. The dual-sheath cytometer can characterize both the submicron picoplankter Prochlorococcus at sample flow rates of 10 mu l min(-1) and, after a 1-min changeover, ultraphytoplankton and nanophytoplankton at sample flow rates >1 ml min(-1). These capabilities make possible real-time analysis of phytoplankton size spectra at sea.