A preliminary assessment of biofouling in a seawater tunnel has been made from a study of the fouling biomass collected from different depths in the tunnel and from the trash racks at the tunnel intake. Fouling biomass was found to be dominated by the green mussel Perna viridis. Data on size-frequency distribution of mussels indicated the possibility of the entry of juvenile mussels through the intake into the tunnel system. It appears that the mussels moved along the length of the tunnel and grew up in size as it reached the fore-bay end. Biomass of green mussels on the trash racks was found to be 57 kg/m' in six months.