Abstract The hypobranchial mucus of Busycon canaliculatum (L.) is surface active when dispersed in sea water. Freshly secreted mucus is adsorbed at the surface in greatest excess after a dilution of about tenfold. The viscosity of the fresh secretion is about 9 cp.; this decreases slightly during measurement but drops no further with increased rate of shear. The viscous properties are associated with the presence of particles 1000–4000 A. in diameter which sediment in 90 min. under centrifugal force 30,200 × g. These particles may be aggregates of smaller particles found in abundance in the supernatant fluid. The solubility and viscous and detergent characteristics of mucus may be helpful in understanding its biological roles.