On-line behavioural monitoring systems are nowadays a common tool in ecotoxicological research. Although applied successfully to freshwater species, they are rarely used in the marine field. Here we present first results about the Marine On-line Biomonitor System (MOBS), a new automatic system for recording behavioural responses of marine and freshwater species. Low power electrical signals are modulated by the behavioural activities of the organisms and then monitored, processed and analysed in real-time. We applied this system to two marine fish species, seabream (Sparus aurata) and turbot (Scophthalmus maximus), and studied their behavioural responses at an acute hypoxic test condition (2 mg O2 l(-1)). The tests were performed for 15 min in small test chambers with isolated fish as well as in large aquaria with groups of six fish. In both cases, MOBS recorded significant alterations in their behaviour: isolated juvenile seabream increased their ventilation frequency (VF) in average by 20%, and grouped fish decreased their swimming activities by 40%. In contrast, isolated juvenile turbot responded to hypoxia with a 36% lower VF. The consequences for data interpretation based on tests for individual fish or groups of fish are discussed.