The behaviour of ragworms,Nereis diversicolor,kept under constant conditions is organized in irregular bouts of activity and rest. Worms usually perform a number of different acts during bouts of activity but the sequences in which they occur are highly flexible. It is suggested that acts, or perhaps groups of them forming motivational systems, “compete”; with one another for ongoing priority. Fluctuations in the levels of causal factors associated with the performance of different acts are probably important in allowing them to replace one another as the ongoing behaviour. Sudden tactile stimulation presented during the performance of an act leads to its interruption; it can do so either temporarily, as when it occurs during irrigation (worms return to this act after the interruption) or permanently, as when it occurs during searching.