The influence of temperature and light on growth and photosynthetic physiology were investigated in embryos of Fucus evanescens grown at 5 or 20°C under irradiances of 15 or 150 μmol photons m−2 s−1 for 7–10 days. Growth was light-independent, but high-temperature embryos were always significantly larger than those grown at low temperature. Photosynthesis-irradiance responses were measured at growth temperature and a standard temperature (20°C) to isolate instantaneous effects of temperature from acclimation responses. Our data indicate that growth and photosynthesis are uncoupled during the early development of Fucus, and that acclimation of the photosynthetic light-harvesting apparatus occurred. Light-limited net photosynthesis (Psub-sat) responded similarly to high temperature and low light. Rates of Psub-sat were similar in embryos grown at 20°C (regardless of light) and at 5°C in low (c. 1.2 nmol O2 mm−3 min−1), whereas those of 5°C high-light embryos were lower (c. −0.04 nmol O2 mm−3 min−1). Changes in Psub-sat were associated with changes in initial slope of the photosynthesis-irradiance curve (α) and dark respiration. Differences in α were attributed to increased absorption due to increased chlorophyll a content and PSII reaction centre densities. Changes in α were also correlated with changes in fluorescence induction kinetics, with high-temperature and/or low-light embryos exhibiting higher ratios of variable: maximum fluorescence (Fv/Fm) than 5°C high-light embryos (c. 0.5 vs. 0.19). In contrast to Psub-sat, changes in light-saturated photosynthesis (Pmax) in response to growth under different temperature/light regimes did not confer metabolic compensation. Rates of Pmax were highest in 20°C high-light embryos (7.3 nmol O2 mm−3 min−1), lower in 20°C low-light and 5°C low-light embryos (c. 2.6 nmol O2 mm−3 min−1) and lowest in 5°C high-light embryos (2.3 nmol O2 mm−3 min−1). We suggest that the ability to achieve temperature-independent rates of Psub-sat may be important for fucoid embryos that recruit in intertidal microhabitats where photosynthesis is often light-limited.