1. Twenty one species, 2 varieties and 4 forms of Sargassaceae were identified in algal samples collected from 60 stations in the Seto Inland Sea and the adjacent waters between 1954 and 1963.
2. The distributional characteristics of these algae are investigated from ecological standpoint in relation to the influence of the offshore water. For this purpose we distinguished the Seto Inland Sea into 4 regions, 2 of which are outer regions and the others inner regions, and the characteristics of each alga are discussed by calculating the percentages of occurrence, R, for each region.
R = s/S×100
where s: number of stations where the alga was collected,
S: total number of collecting stations in the region.
3. As the result of this investigation we may state clearly that Hijikia fusiforme, Sargassum horneri f. furcatodentatum, S. ringgoldianum, S. hemiphyllum, S. micracanthum, S. angustifolium, and Coccophora langsdorfi prefer outer regions which are under the influence of the offshore water, and that H. fusiforme f. clavigerum, S. piluliferum, S. tortile, S. turneri, S. confusum, S. thunbergii f. swatzianum and S. yendoi foster better in inner regions where the influence of the offshore water is rather weak or negligible.
4. The distributions of 32 species, 2 varieties and 2 forms of the same family in the Seto Inland Sea are described on the basis of our collection and the data already reported by other authors.
5. It is hypothetically concluded that while H. fusiforme prefers outer regions, its form f. clavigerum is apt to grow better in inner regions, and also that S. horneri f. furcatodentatum is a form adapted to outer regions though its original species prefers inner regions.