Grazing activity by the dominant zooplankton species in the Oosterschelde (SW Netherlands) was studied as part of an ecological survey accompanying the construction of a storm-surge barrier at the mouth of this tidal inlet. At station WEST, situated at the mouth, potential grazing pressure by the zooplankton amounted to 6% d-1 of the phytoplankton standing stock and 18% of the primary production. At station EAST, in the inland part, 16% of the phytoplankton standing stock and 11% of the primary production was potentially consumed daily. Because the dominant zooplankton species in the inland part (Acartia spp. and cirriped nauplii) tended to feed more on small particles than the dominant species in the seaward part (Temora longicornis and Centropages hamatus), the potential grazing pressure on small phytoplankton (<20 Âµm Spherical Equivalent Diameter) was 3 times higher at station EAST than at station WEST. Ecological consequences of this difference are discussed in relation to developments to be expected in the post-barrier situation.