Abstract. Sporophytes of the brown alga Laminaria saccharina (L.) Lamour grown at 15°C contained significantly more chlorophyll a (chl a) than did similar plants grown at 5°C. The increase in chl a in 15°C plants was due to increased numbers of photosystem II reaction centes, and possibly to increased photosynthetic unit size, compared with 5 °C plants. These changes were associated with increased e values (photosynthetic efficiencies) in 15 °C-grown L. saccharina relative to 5 °C-grown plants. The changes in e together with reduced respiration rates allowed 15 °C-grown L. saccharina to achieve net photosynthesis and light-saturated photosynthesis at a lower photon fluence rate (PFR) than 5 °C plants when both groups were assayed at the same temperature (15 °C). The photon fluence rates necessary to reach the compensation point and achieve light-saturated photosynthesis (I c and Ik, respectively) increased with increasing incubation temperature in L. saccharina grown at both 5 and 15 °C. However, acclimation responses to growth temperature compensated for the short-term effect of temperature on I c and I k. Consequently, plants grown at 5 and 15 °C were able to achieve similar rates of light-limited photosynthesis, and similar I c and I k values at their respective growth temperatures. These responses are undoubtedly important for perennial seaweeds such as L. saccharina, which frequently grow in light-limited habitats and experience pronounced seasonal changes in water temperature.