The flow cytometer, an instrument initially developed for biomedical research, has proven to be a useful and valuable tool in the study of oceanic particles. By using a finely tuned stream flow and elliptical optics (with a laser source) in conjunction with a computer-based data acquisition and display system, the flow cytometer is capable of analyzing several thousand particles (of up to approximately 35 μm diameter) per second. The instrument is designed to allow measurement of several optical parameters on an individual-particle basis. These) parameters include forward light scatter (in two zones from 1.5 to 10 and 10 to 19 ), 90 light scatter and fluorescence at a variety of emission and excitation wavelengths. In addition, the flow cytometer has the ability to sort the sample into subsamples by their optical characteristics. Light scattering measurements have been made on samples of monoclonal unialgal cultures. The capability of measuring the single cell light scatter has allowed for the analysis of variability in 90° light scatter from particle shape effects. Spectral variability in scattering among single-species samples can also be studied. Studies with growing cultures of phytoplankton have demonstrated that the matrix of forward versus 90 light scatter as measured with the flow cytometer may provide a useful method in identifying the physiological state of a sample. A major goal in the use of the flow cytometer for marine optical work is the development of an easily applied algorithm for the rapid determination of the size and refractive index distribution of oceanic particulate matter.