The sargassaceous algae, with their usual habitat in the sublittoral zone where only infrequent studies have been made possible, are not well known on their process of growth. The present article contains some analytical studies on the subject based on the material secured from floating seaweeds collected monthly in the water near northern Kyushu lasting about one year from March, 1957. The major species found floating and dealt with here number 6 species : Sargassum patens, S. serratifolium, S. tortile, S. horneri, S. fullvellum, Cystophyllum sisymbrioides. The growth curve demonstrated by all the 6 species is sigmoid in shape but the study was not made possible to include young individuals measuring less than 0.5 m in length, which are not occurred in the floating material. For the annual species, such as S. horneri and S. fulvellum, there is about three-month-interval between the fruiting period and the occurrence of individual of the small size above mentioned (0.5 m in length), but for the perennial species the same interval is found usually shorter. Other items as to the growth of each species are shown in the following table.【Table】 The process of growth thus observed on the present material may be similar to that of sargassaceous algae growing on the coast of northern Kyushu, where the floating seaweeds probably originate.