The aim of this study was to determine the temperature effects on photosynthesis, growth and reproduction in gametophytes of Alaria esculenta (Linnaeus) Greville and Saccharina latissima (Linnaeus) C.E. Lane, C. Mayes, Druehl et G.W. Saunders from the Artic waters. After 24 days of culture, no gametophytes of either species survived at 20°C. Most growth parameters were greater at 10–15°C than at 5°C. Length and width were similar for both species, but area was greater for A. esculenta and cell number was greater for S. latissima . Female gametophytes were larger than male gametophytes in width and area, but the opposite was observed in cell number. In A. esculenta , but not for S. latissima , the percentage of female gametophytes decreased with increasing temperature. Alaria esculenta female gametophytes produced more sporophytes at 5°C than at 10°C, with no sporophytes at 15°C. In S. latissima , all female gametophytes produced sporophytes at both 5°C and 10°C, with a small percentage of sporophytes at 15°C. Saccharina latissima still had a measurable F v /F m at 20°C, while the F v /F m of A. esculenta was zero at this temperature. Maximum relative electron transport rate (rETR max ) and non-photochemical quenching (NPQ) were constant in the male and female gametophytes of A. esculenta and the male gametophytes of S. latissima at temperatures between 5°C and 15°C. Photosynthesis was saturated at a higher irradiance in A. esculenta than in S. latissima .