Siliceous inclusions play an important role in deterring terrestrial herbivores, but their role in marine systems remains controversial. Sponges are ubiquitous members of marine benthic communities and represent a potentially valuable food source for many predatory species, yet they have few natural predators. An in situ method was used to assess the deterrent effects of siliceous spicules or whole sponge skeleton, which were hypothesised to act as a physical defense in temperate reef sponges. Natural concentrations of spicules from 5 species of sponge and the spongin skeleton from a sixth species were incorporated into artificial diets, which were similar in nutritional quality to the sponge tissues. These were offered to the sea urchin Centrostephanus rodgersii in feeding choice experiments including control discs containing no spicules or spongin skeleton. Spicules from 4 species, Tedania anhelans, Clathria pyramida, Chondrilla australiensis and Chondrilla sp., deterred feeding by C. rodgersii. Spicules from a fifth species,Callyspongia sp., and intact skeleton isolated from Cacospongia sp., failed to deter C. rodgersii. This is the first demonstration of antifeedant activity associated with skeletal elements against sea urchins. Importantly, unlike previous work in the tropics, very small spicules (microscleres) measuring <100 >µm were effective feeding deterrents.