there are disclosed a controlling agent and an antifouling composition against harmful aquatic organisms , which include a particular n - phenylbenzoisothiazolone derivative of the formula : ## str1 ## wherein r &# 39 ; s are the same or different and are independently halogen , alkyl or alkoxy ; n is an integer of 0 to 5 ; x &# 39 ; 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 &# 39 ; s are all alkyl or all alkoxy , n is an integer of 1 to 3 , and when all of the r &# 39 ; 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 .