In the district of the Osima Peninsula, the southwestern part of hokkaido, there are some twenty thermal and hot springs. Of these Yunokawa, Nezaki, Misaki, Oofune, Isoya and Sikabe were already treated in this Acta (1939). This paper informs the thermal algal flora of the rest thirteen springs. The materials forming this subject were collected by the writer mostly in the summers of 1938 and 1939. In the present survey the total range of temperature noted in the springs with algal growth was at 22.8-65.5℃ and that of pH was 2.6-8.5. After careful examination 44 algae were identified. Among them 36 plants belong to the group of Cyanophyceae, I to Chlorophyceae and 7 to Bacillariophyceae. In Siri-uti, a vitriol spring, the water temperature where the algal growth Cyanophycean algae were found in the water. Synechococcus elongatus var. amphigranulatus occurred most dominantly, even in hot water at a temperature 60℃. In Kamaya, an alkaline spring, algal growth was very much limitted and only one Cyanophyceae, Phormidium fragile, was found in water at a temperature 22.8℃ and at a pH 8.5. In Yesan, an acid spring, a peculiar algal vegetation was developed. Only two plants were growing in strong acid water at acidities from pH 2.6-2.8. Cyanidium caldarium occurred in water at temperatures between 30℃ and 32℃ ; Pinnularia Braunii var. amphicephala in water at temp. between 40℃ and 50℃. In Kakkumi, a simple spring, the water temperature ranged from 40℃ to 46.1℃ and its pH from 6.9 to 7.2. here nine species and two varieties of Cyanophycean algae were observed. Of them Chroococcus membraninus, Mastigocladus laminosus and Phormidium laminosum were occurring in unsoiled thermal water, while the rest 8 plants were found only in soiled thermal water. Oscillatoria anguina var. tenella, which is believed new to science, was found in soiled water at temp. 41.3-42.8° and at pH 6.9. In Komano-yu or Komagadake mineral spring, a weak common salt spring, four algae were observed. The plant mass was at 30℃ in the sun, while the mineral water at the vent was at 16℃. Phormidium autumnale was most abundantly growing, accompanied with Oscillatoria formosa and some Diatoms. In Nigorigawa, several hot springs lie scattered. Most of them belong to weak common salt spring except the so-called "Motoyu Spa" which is described as a sulphur spring. Taken altogether the water temperature ranged from 30.5℃ to 65.5℃ and its from 7.7 to 8. In the water of temperature 50-58℃ at "Motoyu", Chroococcidiopsis thermalis, Phormidium laminosum and Ph. luridum were found. The temperature of 65.5℃, observed at the hot spring of Sin-ei-kan Hotel, was the highest record in this study. Mastigocladus laminosus, Oscillatoria Cortiana and Phormidium Corium were growing in this hot water.

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