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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 7-18 degrees C with irradiance at 20, 40, and 80 mu mol photons m(-2) s(-1). The second set combined temperatures of 3-13 degrees C with irradiance of 10, 100, and 200 mu mol photons m(-2) s(-1). 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 8-13 degrees C at the lowest irradiance of 10-mu mol photons m(-2) s(-1). Light had a significant effect on both male and female gametophyte growth only at the higher temperatures. Temperatures of 8-15 degrees C and irradiance levels of 10-100 mu mol photons m(-2 )s(-1) were conditions for the highest sporophyte growth. Sporophyte and male gametophyte growth was reduced at both temperature extremes-the 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.