In order to examine effects of seawater temperature on the growth of Porphyra foliose thalli, outdoor raceway tanks with circulating water were newly designed, and provided the experimental condition simulating Nori cultivation farms in the sea. Frozen seed nets of Porphyra yezoensis form. narawaensis were set in the raceway tanks. During the period of the two experiments, December in 1985 and February in 1986, mean seawater temperature ranged from 8.0 to 23.4°C. Standing crop of foliose thalli increased normally at 10-17°C, but the increase rate of standing crop lessened at the temperature higher and lower beyond these limits. Visible absorption spectra of living foliose thalli, indicating concentrations of photosynthetic pigments, did not vary much within the range of seawater temperature from 8 to 20°C. The optimum seawater temperature for the growth of Porphyra foliose thalli ranged from 10 to 17°C in the raceway tanks. This temperature range confirmed results obtained through in vitro experiments but was higher than that in Nori cultivation farm.