An experimental study was made of the life cycle of a large-sized brown alga Sargassum muticum and its control using simple batch cultures in order to establish a routine system of supplying embryos as materials for laboratory experiments. Growth of early germlings was faster at higher temperatures under a long-day condition (15h L). On the other hand, main branch elongation was promoted under a short-day condition (9h L). Under the long-day condition, growth of leaves was active while main branch elongation was very poor. Under the short-day condition, main branches extented to form vesicles. Receptacle formation was observed by transferring the thalli grown and differentiated under the short-day condition at 10-15°C to the long-day condition at 20°C. Fertilized eggs repeated cell divisions to form early embryos on receptacles. The embryos were then liberated to the bottom of culture vessels and grew into normal thalli. By controlling temperatures and photoperiods, it became possible to regulate the growth and differentiation of the species, which brought a small thallus only 5.6cm high into fertility.