Abstract: 1. After studying the cultures of unicellular origin for more than two years, we have determined two species of Platymonas collected at Tsingtao--a new variety of Pl. helgolandica Kylin, i.e., Pl. helgolandica var. tsingtaoensis Tseng et T. J. Chang var. nov*., and Pl. subcordiformis (Wille) Hazen. As compared with Pl. helgolandica var. helgolandica (with 2 to 9 stigmata), the new variety has, generally, only one stigma, and rarely 2 to 4; while the overall size varies more greatly, its length being from 16 to 30μ. 2. The size and shape of both Pl. helgolandica var. tsingtaoensis and Pl. subcordiformis are greatly affected by their living conditions and the flatness of their body shape is a matter of relativity. For instances, when cultured in fresh culture solution and favourable temperature, and under the condition in which the cell division is more rapid, they are smaller and flattened, even have a dorsoventral distinction, but while in old culture solution and low temperature, and under slow or non-dividing condition, they are larger and the transverse view of the body gradually becomes roundish. 3. The stigma position of Pl. helgolandica var. tsingtaoensis is inconstant. When smaller, the single stigma lies generally near the pyrenoid, but after having grown old and large, it shifts to the anterior part of the body. The multi-stigmatic phenomenon oc- curs only in the period of transition from the smaller to the larger. On the other hand, the stigma position of Pl. subcordiformis is more constant and always adjacent to the pyrenoid. 4. Cyst formation occurs in both species when the culture becomes old, but on experiment with Pl. helgolandica var. tsingtaoensis for more than one year, it proves that if the organism is continuously maintained in fresh culture solution, it will divide and redivide, but the cell never becomes large and no cyst formation occurs. 5. The rotation and, eventually, the inversion of the daughter protoplasts during cell division, as having been already well described by Zimmermann (1924), occur constantly in both species, and in fact, the strongly flattened and dorsoventral structure of the young Platymonas is principally owing to the inversing behaviour of the daughter protoplasts and the moulding influence of the mother cell wall. 6. In the old culture of Pl. helgolandica var. tsingtaoensis, a rather rare and strange phenomenon was observed, i.e., some individuals were joined together in pairs in various manners while still moving about freely. This is described and interpreted here rather tentatively. 7. The limitation between the genera Platymonas and Pyramichlamys and that between the species Pl. subcordiformis, Pl. tetrathele and Pl. helgolandica are discussed also.