The effects of steady state nitrogen limitation on the optical properties of Dunalielh tertiolecta were investigated. Growth rate was varied in a continuous culture under constant irradiance and temperature with absorption, fluorescence, and cellular characteristics including pigment contents determined at each steady state. The cellular concentration of Chl a increased with growth rate while Chl-a-specific absorption and fluorescence both decreased. In addition, the quantum yield for growth varied by more than a factor of 3.5 over the growth conditions examined, with the highest yield in the most rapidly growing cells. The decrease in magnitude of Chl-a-specific optical properties is caused by pigment package effects and changes in the abundance of accessory pigments relative to Chl a. Changes in absorption and fluorescence properties are consistent with theoretical predictions for discrete packages. In addition, pigment-based reconstruction techniques overestimate the magnitude ofboth in vivo and unpackaged absorption by D. tertiolecta under these growth conditions. The observed variability in Chl-a-specific absorption and quantum yield is a fundamental aspect of phytoplankton physiology that should be incorporated into models of oceanic primary production.