Changes in the photon absorptance of algae were measured using an integrating sphere. Exponential growth-related increases in absorptance with time were found for all the algae studied except at the lowest (4 μmol m-2 s-1) light level. The highest absorptances were found at 40 pmol m-2 s-1. Changes in photon absorptance were paralleled by changes in chlorophyll a content, however, the relative changes were not always quantitatively similar. Anabaena variabilis Kütz. exhibited higher absorptance than Scenedesmus obliquus Pringsheim at all photon fluence rates (PFR) studied, the relative difference varying with incident PFR. Progressive increases in absorptance were not found with S. variabilis at PFRs less than 40 pmol m-2 s-1 although higher values were recorded at 4 than at 10 pmol m-1 s-1. In contrast, S. obliquus showed a progressive decline in absorptance from 40 to 10 pmol m-2 s-1. Approximately linear increases in dry matter, protein and chlorophyll a were found with increasing accumulated PFR, particularly when results were expressed on a relative accumulated absorbed PFR basis. Transfer experiments suggested that S. obliquus responded to the total quantity of photons absorbed whereas the response of A. variabilis was dependent upon light regime.The difference in the growth response of A. viriabilis may be due to the saturation of the dry matter/absorbed photon relationship at relatively low light levels. Growth efficiency (GE) increased for S. obliquus and A. variabilis with decreasing light level irrespective of daylength and despite differences in the response of these algae to variations in PFR. Anabaena viariabilis had higher GEs than S. obliquus at lower PFRs whereas S. obliquus had slightly higher GEs at higher PFRs. Overall, A. variabilis appeared to be capable of better growth at low PFRs than S. obliquus, however, the restricted ability of this cyanobacterium to respond to changes in PFR, particularly on a L: D regime may be a disadvantage in the fluctuating light climate of the natural environment.