PURPOSE: To provide an antifouling paint with excellent self-abrasive properties and reduced frictional resistance, free of adherent slime, by incorporating a specific organotin polymer.

CONSTITUTION: An organotin polymer of formula I {wherein R is 6W20C straight-chain or branched alkyl (phenyl); R' is H, 1W4C lower alkyl; R" is H, 1W4C lower alkyl, phenyl, carboxyl (ester); R''' is 3'6C lower alkyl, phenyl, benzyl, allyl; the structural unit -[M]- comprises one or more unsatd. monomers capable of copolymerizing with formula II, and the unit -[M]- and formula IIIare randomly arranged in the molecule; (a+b) is 10W1000; b/(a+b) is 0.5 or above} and, if necessary, cuprous oxide, chlorinated rubber, etc. are incorporated to obtain the titled paint.