PURPOSE: To obtain the agent which provides antifouling effects over a very long term and is esp. effective in controlling harmful marine organisms.

CONSTITUTION: The agent contains an inclusion compd. comprising an isothiazolone compd. and 5,5'-dichloro-2,2'-dihydroxydiphenylmethane. Slowly releasing the isothiazolone compd., the guest compd., into water, the agent maintains its antifouling activity over a very long term. It releases the compd. over a very longer term than an antifouling agent contg. a conventionally used inclusion compd. esp. in sea water and hence is very useful for a ship bottom coating, an antifouling agetn for a fishing net, etc., in controlling slime.