It is important toselect suitable organisms to adapt the requirement of different environment monitoring purposes.Following our previous study,the behavioral responses of Daphnia magna and Japanese madaka(Oryzias latipes)were investigated and compared under flow-through conditions in an on-line biomonitoring system.The results showed that both D.magna and Japanese madaka had similar biological clock,but the circadian rhythms of Japanese madaka was more clearly recorded than that of D.magna.And the sensitivity of D.magna was about two orders of magnitudes higher than that of Japanese madaka in different types of toxic chemicals (dichlorovos,deltamethrin and cadmium chloride).However,when both animals were used in an on-line biomonitoring system,the life span ofD.magna was less than 7 days and Japanese madaka could last for more than one month without feeding.Therefore,D.magna was proposed to be a more sensitive bioindicator and was suitable for short term monitoring the pollution events at concentration level closing to the water quality standard,while Japanese madaka was more suitable for the long-term monitoring for accidental discharges.