In September 2008, organotin (Ot) compounds were prohibited from being used worldwide. From 1997 onward in Japan, the production of paints containing TBT (tributylin) compounds was prohibited, and thus alternatives to Ot antifoulants have been used since then. It has been said that the decomposition characteristics of these materials are better than those of Ot compounds. The toxicity of alternative Ot antifoulants (e.g., diuron, irgarol 1051®, and Sea-Nine 211®) and Ot compounds (TBT and TPT (triphenyltin)), using oysters that inhabit a large area of Hiroshima Bay, were evaluated. The results showed that the toxicity of diuron and irgarol 1051 is very low, and the toxicity of Sea-Nine 211 is almost the same as that of TPT. Sea-Nine 211's effect was stronger on oysters than other shellfish, causing concern about the extent of Sea-Nine 211's impact on oyster development.