The life cycle of the cyanobacterium Aphanizomenon flos-aquae was elucidated based on data concerning the growth, formation and degradation of the bloom, as well as consideration of its mechanism. Abiotic factors such as water temperature, pH and day length are of critical importance in determining not only the growth in the water, but also germination and formation ofakinetes. Besides these abiotic factors, biotic factors such as competition between coexisting phytoplankton and grazing byzooplankton should be taken into consideration-particularly at the initial and final stages of the bloom.