There are disclosed a controlling agent and an antifouling composition against harmful aquatic organisms, which include a particular N-phenylbenzoisothiazolone derivative of the formula:

wherein R's are the same or different and are independently halogen, alkyl or alkoxy; n is an integer of 0 to 5; X's are the same or different and are independently halogen, nitro, alkyl or alkoxy; and m is an integer of 0 to 4; with the proviso that when the R's are all alkyl or all alkoxy, n is an integer of 1 to 3, and when all of the R's are different, n is an integer of 2 or 3. Also disclosed is a method for preventing or inhibiting the adhesion of harmful aquatic organisms to water-exposed articles, which includes applying the antifouling composition to the water-exposed articles.