Abstract: Although the utilization of the various species of Laminaria in various parts of the world has a long history, and the cultivation of one of the species (L. japonica Aresch. vernacularly called “haidai”) has been practised in China for some thirty years, the industry having developed tremendously since 1950, yet the genetic study of this marine alga, and indeed of nearly all other algae has not yet begun. No selection work has been done in the haidai cultivation industry. Parke (1948) has done extensive works on the growth of Laminaria saccharina (L.) Lamour. and Tseng, Wu and Sun (1957) have carried out similar studies on Laminaria japonica Aresch. The present paper describes a new line of work begun in 1958 on the haidai, resulting in the breeding of a new breed named Haiqing No. 1 of Laminaria japonica. The original materials used in this breeding work were all summer sporelings of the haidai cultivated at low temperature in the summer, 1958. The methods of laboratory and sea cultivation of Laminaria described by Tseng etc. (1957) were adopted. As different fronds of Laminaria under similar conditions vary in regards to growing rate, size, and degree of natural casting away of distal frondal tissues, and as sexual process together with crossing takes place regularly every year during reproductive season, the present Laminaria populations under cultivation are assumed to possess a high level of hibridity. It follows that the main breeding work on such a population should be inbreeding plus selection. Inbreeding was carried out by using single mature sporophytes to yield spores. Selection was performed on gametophytes and sporophytes in three steps: (1) cultivating gametophytes under 20 ℃ or 22 ℃ for more than one month to kill those gametophytes unable to resist such high termeratures; (2) selecting fast growing spore- lings for transplantation, and (3) selecting healthy and large sized mature sporophytes to yield spores for further cultivation. From four years work with successive inbreeding and selection, a new Laminaria population has been obtained. This is the beginning of a new breed, Haiqing No. 1. The breeding work has been carried out at Tsingtao, 36˚ North Latitude, in the Institute of Oceanology, Academia Sinica. The new breed of Laminaria differs statistically significantly in several important characters from the control population: it grows faster, and matures much later, being more resistant against higher temperatures, and having a longer growing period; it has longer and broader blade with a longer stipe and is heavier in weight. The new breed has been tested in Amoy in south China, 24˚ 30′ North Latitude, and similar results have been obtained. Tables 1–4 and figures 2–4 give the main results of the various experiments. Figure 1 shows diagrammatically the history of the new breed. The results have been analysed from geneticat point of view. The data do not support the hypothesis of directed variation or that of induced mutation. It is argued that the new breed has been produced through the interaction of the following factors: (1) the hybrid individual sporophytes which possess rich genetic basis being the object of selection, (2) inbreeding which is naturally followed by segregation and recombination of genetic factors having given rise to a variety of forms, and (3) selection which controls the direction of development of the population having given rise to the type of population we wanted. It is the successive inbreeding and selection on the hybrid genetic factors which gave the new breed. The inheritance of the length of the stipe has been analysed for the first time. This is surely an inheritable character, although it is influenced by some environmental condition such as the intensity of light. Treating gametophytes with sufficiently high temperatures appeared to have little effect on the length of the stipe. Tables 5–7 summarize the materials regarding the inheritance of the stipe character. We are greatly indebted to Dr. C. K. Tseng, the vice-director of the Institute of Oceanology, Academia Sinica, who first noticed the significance of the new line of research and gave us stimulating interest and encouragement throughout the course of the investigation.