AbstractEnvironmentally friendly organic-inorganic hybrid materials with repellent activity against marine fouling organisms, such as blue mussel, have been developed using interpenetrating polymer networks (IPNs), composed of a 3-dimensional siloxane matrix of tetraethoxysilane (TEOS) and poly(vinylacetate) (PVAc). Facile bioassay using blue mussels allowed screening the adequate proportions of those components. The repellent activity of IPN with PVAc/silica reached approximately 90% at a specific fraction, relative to that of tributyl tin oxide (TBTO). On the other hand, poly(methylmethacrylate)(PMMA) /silica composite did not show marked repellent activity against blue mussel, even though chemical structure of MMA is similar to that of vinylacetate.