Abstract: Biological characteristics of Planktonema were systematically studied by observing its life cycle,microstructure,ultrastrutture and cell division. Planktonema is unbranched filament in a row of cell. The cells are cylindric or elliposoid,mainly joined by cylindric cellulose cell walls and the cells contain chlorophyll a,b and β-carotene. Under transmission electron microscope. the cell walls clearly stratify and mostly have 2～3 layers. The chloroplast is lamellar, parietal and usually two or several thylakoids overlap;there are 1～2 pyrenoids in chloroplast and the pyrenoids have envelopes of starch grains,sometimes one or several thylakoids penetrating the pyrenoid. 1 ～2 mitochondrions stand by the chloroplast, and the ridges do not branch. Golgi complex sits near the cell circumference and excretes many vesicles. One nucleus is irregular square or globular, close to one pole of the cell. One or several vacuoles,which present bright spots at both poles of the cell under light microscope,usually contain some material dyed deeply. The studies on the morphology, the cell division and the life cycle demostrate the cells have not desmochisis and by the cell division the mother cell produces two autospores with their own complete cell walls. Aseries of autospores are linked to become the filamentous forms by the remaining, discontinuous cylinder-shaped or cup-shaped mother cell wall, and the formation and development of the filaments is fulfilled by asxeual reproduction, therefore, this kind of filaments naturally are the pseudo-filaments.