The Multispecies Freshwater Biomonitor (MFB) based on quadropole impedance conversion technique is a novel biological early warning instrument for online water quality biomonitoring. The MFB has been tested with Gammarus pulex (24 individuals) and Daphnia magna (16) exposed to process water in a drinking water processing plant in the Netherlands. After 2 days of acclimation, an acid pulse with 10 M HCl (duration of 6 h) was simulated. Subsequently, fresh process water was reconnected to the MFB for the following 3 days of recovery. The responses of the two crustaceans (survival, time to death, activity patterns) were recorded online during the experiment. After this laboratory experiment, the MFB was tested in situ with surface water for ca. 2 months. The results will be discussed and compared to those of other biomonitors.