The PI's request funding to develop a commercial detector for identifying and counting marine plankton that will be integrated into an instrument for extended use at sea. The micro-plankton detector will be based on the standard detector that Cytopeia uses in its flow cytometers and cell sorters. This detector will be supplemented with position sensitive detectors (PSDs) that sense the position of particles that flow through its observation area. Using well-established principles of flow cytometry and confocal microscopy, the detector will be able to discriminate between particles that flow through the system's optical axis and those particles that pass out of focus. The system will process fluorescence signals from in-focus particles only, and will ignore out-of-focus events that cannot be measured accurately. This will result in will a reliable in situ instrument for routine cytometric characterization of phytoplankton populations.