In modeling the optical properties of and radiative transfer within ocean water, a common assumption is that the optical properties of the various dissolved and particulate constituents per unit concentration are constant. While this assumption is probably valid for non-living materials, it is frequently not the case with phytoplankton. Living cells are dynamic in their size, shape, and composition and respond readily (time scale of minutes to hours) to changes in the environment. Since each of these factors plays a role in determining the light scatter and absorption properties of the cell, the assignment of optical constants to phytoplankton cells, especially within Case I waters, may introduce significant errors in any theoretical treatment of radiative transfer and may result in erroneous interpretations of bulk optical measurements. Single-cell light scatter and beam attenuation of several species of marine phytoplankton, Thalassiosira pseudonana (clone 3H), an unidentified prasinophyte (clone OMEGA 48-23), Paviova sp. (clone NEP), Emiliania huxleyi (clone BT6), and an unidentified cryptomonad (clone 1D2), respond rapidly to increased light intensities. Flow cytometric determinations of cell refractive index n, measured at 488 nm, was found to decrease under high light conditions while cell size D, expressed as equivalent spherical diameter, increased. Beam attenuation c, measured at 660 nm, was generally found to increase when cultures were exposed to high light intensities. With a constant cell concentration, the observed change in n tends to decrease beam attenuation, while an increase in D tends to increase c. The magnitude and sign of dc/dt will depend upon the relative change in mean cell refractive index ii and the mean cell size i In response to bright light, i5f 3H and NEP was found to increase more rapidly than particulate organic carbon (POC). POC normalized to total cell volume increased within NEP and decreased within 3H', while at the same time, decreased within both cultures, suggesting that the cells swelled. As a result of light induced changes in i and ! optical properties of cells can change independently of biomass.