The effects of photoacclimation on growth, photosynthesis, pigment content and elemental composition of Ulva curvata and Ulva rotunda& which grow together in eutrophic areas of southern Spain, were investigated. CuItures were grown for 6 days at different photon fluence rates (PFR) ranging from darkness up to 200 #mol m -2 s-1 under nutrient-sufficient conditions (artificial seawater supplemented with ammonium and phosphate). Growth rates were not light-saturated (up to 200 #mol m -2 s-l), reaching a value close to 0"2 d -I. Growth rates based on mass, area or C content were equivalent, except in darkness and very low light levels (2 #molm -2 s 1), where thallus expansion occurred by diluting internal biomass and C. Chlorophyll and absorptance showed a bell-shaped PFR-response curve, with maxima at 30-60#molm -2 s 1 and lower values at light saturation and under light-limiting conditions. Although net NH4 + incorporation was not affected by growth-PFR, there was inefficient assimilation of N at low light levels, which may restrict chlorophyll, protein and membrane synthesis. The light-saturated photosynthesis rate (Pm~x) displayed a bell-shaped PFR-response curve, when expressed on an area basis, whereas it was saturated from 1I #molm 2 s-1 when scaled to internal N. This suggests that ribulose-I,5-bisphosphate carboxylase/oxygenase activity could be the rate-limiting step underlying Pm~x for high-light-acclimated algae, whereas electron transport chain elements may limit Pmax under light-limiting conditions. Pma~ and tissue C were always higher for U. curvata. Dark respiration rates were positively correlated with growth rates, and photon yield of net growth declined with increasing growth-PFR. The results are also discussed in relation to cell size, since U. curvata cells are smaller than those of U. rotundata.