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The present authors here propose to establish a new genus within Scenedesmaceae. The alga was collected from Matsue City of Japan in August 1960. With careful observations on the life history, the authors have concluded that the present alga is a member of a new genus that comes nearest to Scenedesmus and Tetradesmus of Scenedesmaceae. So far as the arrangement of cells is concerned, the outline of colonial organization of this alga is similar to that of Sorastrum. But there are neither connecting mucilaginous strands nor mucilaginous enveloping common matrices. The colonies of the present alga are composed of several four-celled units and each of these units is originated respectively from a single mother cell by means of autospore formation. With continuous careful observations of its natural population, it was clarified that this alga showed two morphological patterns in its life cycle as mentioned below. At first a four-celled coenobial unit is formed from a single mother cell by its transverse division. The four-celled coenobial unit thus made lies always in one plane and all cells are compactly connected. This stage is named " Scenedesmus stage " by the authors. The four cells of this flat coenobial unit grow to be rearranged into a solid bundle just like that of Tetradesmus. In this stage cell walls are never adhered together, but the cells are loosely connected with each other. The authors give a name " Tetradesmus stage" to this stage. After the repetition of both autospore formations and bundle rearrangements, the mature compound coenobia are built up. The " Scenedesmus stage" and the " Tetradesmus stage" successively occur in April to May. The formations of compound coenobia begin in May to July and the mature vegetative colonies can be encountered till November. The decomposition of colonies and the segregation of each cell from a compound coenobium supposedly occur in winter to early spring. Though this alga agrees with the genus Tetradesmus in arrangement of cells in two planes with their longitudinal axes parallel, this alga has significant differences, viz. 1) mature colonies are compound coenobia which are composed of several fourcelled units or daughter coenobia, and 2) each cell of the four-celled unit is connected loosely. Therefore we wish to establish a new genus Pseudotetradesmus that is closely related to Scenedesmus and Tetradesmus. Pseudotetradesmus Hirose et Akiyama gen. nov. Coenobium planctonicum, juvenaliter semper 4-cellularis, 2-4 congestum cum haec adultum sunt. Omnis cellulae juvenalium coenobiorum quattuor-cellularium in una plana collocatae, nuper in et duabus planae et multo laxe conglobatae, et axes longitudinales omnium quattuor cellularum paralleli. Progeneratio semper autocoloniis. Coenobium free-floating, composed of assemblage of 4-celled coenobial units. Each cell of 4-celled unit arranged in one plane when young, and later arranged in two planes with their longitudinal axes parrallel, and loosely connected together

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