An automatic recording apparatus for measuring the filtration rate in suspension-feeding bivalves is described. The concentration of algae in the experimental medium is kept constant throughout each experiment by addition of Phaeodactylum tricornutum from a chemostat. Within the range of body size 5.7 to 283 mg (W=dry weight of tissues), the filtration rate (F=ml min-1) at 15°C in Mytilus edulis L. follows the allometric equation F=0.85 W0.72. Within the concentrations 0.18 to 0.70 mg algal dry weight l-1, the filtration rate in mussels of 132 mg dry flesh weight ranges from 33.1 to 41.0 ml min-1. At 0.18 mg algal dry weight l-1 the mussels filter continuously for 20 h, with a high constant rate that presumably represents the water transport capacity under optimal laboratory conditions.