Online biomonitors register biological effects of toxic discharges on selected indicator species and allow for fast, continuous, and ecological relevant water quality control. This paper presents a new online biomonitor based on a quadropole impedance conversion technique that records simultaneously several behavioral parameters of Gammarus pulex, a new biomonitoring test species. The behavior of two different populations of G. pulex was compared; one population originated from an anthropogenically unpolluted stream, and the other population lived at the copper-polluted study site, where the biomonitor was placed. Responses to simulated copper pollution peaks of 70 μg of Cu2+/L were registered in the biomonitor and compared to the natural drift. G. pulex was the most abundant species in the natural drift. A nocturnal drift maximum was found in the natural drift and for both popula tions in the biomonitor. Mortality was high in the biomonitor; the local population survived slightly better than the reference population. The reference population showed significantly less activity than the local population measured as number of active organisms per day and time spent on locomotion and ventilation. Copper pollution pulses provoked increases in number of active organisms and time spent on locomotion in the biomonitor; however, no significant changes in the natural drift were registered.