Pairs of fish of the South American "pulse" species (of 40-100 Hz frequency), Rhamphichthys rostratus, were studied for position and motor behaviour in tanks fitted with hollow tiles on the bottom. Each pair consisted of a larger and a smaller fish. The larger fish in each pair seemed to be territorially dominant and tended to stay inside the tiles more than the smaller one; the smaller fish moved around more than the larger one. All animals stayed more frequently inside the tiles during the daytime period. During rainy weather, or when each fish was studied separately rather than in pairs, the animals stayed more frequently inside the tiles, usually resting for long periods of time. The larger fish in each pair spent more time on its side than the smaller one, showing a diurnal rhythm in the amount of time spent in this position, whereas no such rhythm was observed for the smaller fish. The acceleration of the frequency of electric organ discharges (EOD), called "jump", and social interaction were also recorded. Smaller fish showed more jump activity than larger ones. The occurrence of jumps was related to the anti-parallel and tandem position of the two fish, seemed to increase with time of settled position and favourable temperature, was more likely to occur when the individuals were quite close to each other and to decrease during rainy weather.(ABSTRACT TRUNCATED AT 250 WORDS)