Growth and photoadaptation were measured in Ulva lactuca discs kept in the laboratory at cont~nuous photon flux densities between 0 and 55 km01 m-' S-' a t 7 "C. The light compensation point for growth (I,) was ca 2.5 pm01 m-'s-' and was similar to the minimum hght compensation point for photosynthesis of U. lactuca. I, corresponded to only 0.5 % of surface light In Danish waters during summer and 2 to 4 % at 7°C in autumn. Photoadaptation was regulated mainly by changes in Light absorptance and dark respiration rates whereas quantum efficiency was approximately constant. Absorptance was highest at intermedate light levels (2.5 to 8.8 pm01 m-'s-') and coincided with maximum efficiencies for growth (ca 0.024 m01 C fixed per m01 incident photon and 0.056 m01 C fixed per m01 photon absorbed). Dark respiration rates were linearly related to growth rate and doubled from low to hlgh Light accllmated plants. Respiratory maintenance costs, estimated from the carbon growth balance of non-growing U. lactuca in very weak light (<0.3 pm01 m-' S' ) , were low at 0.0094 to 0.012 molC respired (m01 cell C) ' d ' (half-life of cellular carbon = 58 to 74 d) and corresponded to the experlmentally denved values. These low maintenance costs are essential to maintain a low I, value and would also allow U lactuca to survive extended periods in the dark. The results do not conform with the previous vlew of Ulva lactuca as a classic ruderal sun species, but show a high capacity for growth In the shade.