Anthropogenic disturbances, including coastal habitat modification and climate change are threatening the stability of kelp beds, one of the most diverse and productive marine ecosystems. To test the effect of temperature and irradiance on the microscopic gametophyte and juvenile sporophyte stages of the rare kelp, Saccharina angustissima, from Casco Bay, Maine, USA, we carried out two sets of experiments using a temperature gradient table. The first set of experiments combined temperatures between 7â18°C with irradiance at 20, 40, and 80 μmol photons mâ2 sâ1. The second set combined temperatures of 3â13°C with irradiance of 10, 100, and 200 μmol photons mâ2 sâ1. Over two separate 4-week trials, in 2014 and again in 2015, we monitored gametogenesis, the early growth stages of the gametophytes, and early sporophyte development of this kelp. Gametophytes grew best at temperatures of 8â13°C at the lowest irradiance of 10-μmol photons mâ2 sâ1. Light had a significant effect on both male and female gametophyte growth only at the higher temperatures. Temperatures of 8â15°C and irradiance levels of 10â100 μmol photons mâ2 sâ1 were conditions for the highest sporophyte growth. Sporophyte and male gametophyte growth was reduced at both temperature extremesâthe hottest and coldest temperatures tested. S. angustissima is a unique kelp species known only from a very narrow geographic region along the coast of Maine, USA. The coupling of global warming with high light intensity effects might pose stress on the early life-history stages of this kelp, although, as an intertidal species, it could also be better adapted to temperature and light extremes than its subtidal counterpart, Saccharina latissima. Key words: gametophyte; irradiance; kelp; north Atlantic; Saccharina angustissima; sporophyte; temperature