The yield of in-vivo fluorescence per unit of cellular chlorophyll varies markedly according to phytoplankton species and physiological state, and is also highly sensitive to the configuration of the measuring equipment. This means that great caution has to be exercised in the use of fluorescence sensors for in-situ monitoring of chlorophyll concentrations. On the other hand, the sensitivity of fluorescence yield to biological parameters raises the possibility of combining fluorometry with other optical measurements to produce new probes for monitoring the adaptive response of phytoplankton populations to their changing environment.