Seasonal changes in thallus length were investigated in populations of two dominant temperate Sargassum species, S. okamurae and S. micracanthum, on a rocky shore off Kutsuu in Tosa Bay, southern Japan from February 2006 to March 2007. Maximum thallus length in S. okamurae appeared in August 2006, at which time, seawater temperature was the highest of the year. Maximum thallus length in S. micracanthum occurred in December 2006. Receptacles were produced after termination of vegetative growth in both species. Apical segments were taken from wild thalli of S. okamurae and S. micracanthum in October 2005 and March 2006, and they were cultured under various temperature conditions. The optimal temperatures for growth of S. okamurae segments in October were 15-30℃, though that in March was at 25 ℃. The relative growth rate in the October segment under the optimal temperatures was 1.5-2.2-times greater than that in the March segment. The optimal temperatures for growth of S. micracanthum segments in October and March were 20 and 25℃, respectively. The relative growth rate in the October segments was 3-times higher than that in the March segments. Seasonal growth patterns in S. okamurae and S. micracanthum are discussed, based on results from the present culture studies and previous field studies.