- Background Fish are a diverse group of vertebrates with very different brain structures. The study of the neurobiology of fish can thus lead to many important insights on information processing in the brain in a variety of environments. New method We developed a novel wireless technique to record extracellular neural signal activity in freely behaving fish. The system is based on a data logger and enables continues recording of up to 2.5 h. Results For proof of concept we recorded from the optic tectum of goldfish. We found correlations between the activity of the optic tectum and the light intensity in the room, as expected. Comparison with existing methods Due to the technical difficulties involved in making electrophysiological recordings of behaving aquatic animals, there is no method for recording electrical neural activity from the brain of freely swimming fish. Conclusions This powerful tool should facilitate studies of information processing in behaving fish and other behaving aquatic animals.