The rate of secretion and number of byssal threads by which blue mussels (Mytilus edulis L.) reattach were measured in circulating seawater and with the mussels exposed to currents between 10 and 20 cm/s. In circulating water, larger mussels secrete more byssal threads than small mussels and appear to do so more rapidly. If detached blue mussels are exposed to currents, this pattern is reversed; when exposed to high currents, small mussels reattach more rapidly than large mussels.