Flow cytometry is used to measure dilution-induced changes in the optical properties of Dunaliella tertiolecta-light scatter, cell fluorescence, and refractive index. Observed changes in cell optical properties are compared to simultaneous measures of cell volume and count, extracted chlorophyll a concentration, and the (14)C uptake rate. Flow cytometric measurements reveal short-term dilution effects (within 1 h of dilution) that, are not evident in other morphological or physiological measurements such as cell volume, extracted chlorophyll a concentration, and (14)C uptake rate. Data are presented which suggest that these short-term changes in cell optical properties are the result of changes in the real component of refractive index, possibly due to a rapid and temporary rearrangement of the internal cellular structure. Long-term changes are observed in time series measurements of cell volume and count, extracted chlorophyll a concentration, and (14)C uptake rate.