From the gametophytic blades of Porphyra yezoensis Ueda which developed from conchospores and aged 10 to 80 days by culturing in the laboratory, the single cell were enzymatically isolated and regenerated into plants showing ten developmental types, such as normal blades, abnormal blades, cell2masses, sexual cell2masses and others. They did not only have the different morphological and structural features, but also were different in cell size and cell arrangement, monospore2release and their final developmental results. When the single cell were isolated from the mother blades in different ages, the numbers and the percentages of their developmental types were different, respectively. When the age of the mother blades increased from 10 to 30 days, among the plants regenerated from the single cell, only a few of cell2masses appeared, and the percentages of the normal blades and abnormal blades with rhizoids sharply decreased, but the percentage of the abnormal blades without rhizoids increased. When the mother blades aged more than 40 days, among the plants regenerated from the single cells, the normal blades did not appear, and the percentages of the abnormal blades without rhizoids also sharply decreased in addition to the significant increase of cell2masses, and the percentages of both spermatangia and caposporangia gradually increased with the increase of the ages of mother blades. When the single cells were isolated from different parts of a long blade, among the regenerated plants from them, the percentages of both normal and abnormal blades significantly decreased, but with increasing of percentage of the cell2masses. These results suggested that the single cell isolated from the gametophytic blade of P. yezoensis regenerated into different types of plants in vitro are attributed to the difference of the differentiation stages of the cells in vivo. The differentiation period of the gametophytic blade cells from the conchospore cell to sexual cell, can at least be divided into eight differentiation stages.