The present studies were carried out to establish the early warning systems with waterflea. In order to monitor some toxic compounds that might be inflowed Seongseo industrial stream, we adopted Daphnia as a biological toxicity test animal. Daphnia magna was fed on unicellular algae, Chlorella, in this study. Structure of D. magna was observed daily by microscope. The mating of D. magna could be photograhed with Microscope Camera. Chemical analysis of the industrial effluents water quality was performed. Except for pH, water quality of test sites was over the level of environmental water quality standard. Before the biomonitor was installed in field, we have sampled from the industrial stream and then applied the sampled waters to the biomonitor in laboratory by 2∼3 day intervals in a week from April, 1995. In May, the average impulse was 42, much lower than those of June and July, and most of test organisms died, so the average minimum impulse was almost zero. In June and July, the level of impulse tended to become increase with the recovery of water quality, and the swimming activity of Daphnia was generally stable. Also, the average maximum impulse was near 300. The average impulse of August was lower than that of July and swimming activity of Daphnia to the water quality was regular and stable. In November, we began to apply the industrial sream water to the biomonitor which was installed at the outlet stream of Seongseo Industrial Complex. Based on the overall results, it was seemed that the swimming activity of Daphnia was affected much by complex toxic compounds, not by a single toxicant.