Decomposition of Anabaena flos-aquae and Chlorella vulgaris in a dark condition was examined. Change of pH. DO, heterotrophic plate count, protozoa number, dissolved ortho-phosphorus and dissolved total phosphorus during the algae decomposition was also determined. The results are as follows: Although 80% of Anabaena cells were decomposed after 8 days, the Chlorella cells were decomposed only 13% after 9 days. The decomposition rate was markedly different with the algal species. Aerobic bacteria and protozoa play a important role in the decomposition of algae even though under the closed system. The change of pH during the decomposition of algae was negligible. Meanwhile, DO was sharply decreased in the initial stage of the decomposition and progressively decreased in the latter stage. Phosphorus concentration in the solution increased with the decomposition of algae. Especially the dissolved total phosphorus concentration increased abruptly with the lysis of protozoa, the number of which had increased previously with the decomposition of algae.