Abstract: A marine anti-fouling paint capable of becoming smoother in use on a ship's hull moving through sea water comprises an effective amount of marine biocide and, as the film-forming binder, a polymer containing diorganotin moities linked to the polymer by carboxylate linkages. The binder is preferably a copolymer of units of a diorganotin salt of an olefinically unsaturated carboxylic acid of the formula: ##STR1## where R' is an olefinically unsaturated group, each group R is a monovalent organic radical linked to the tin atom by a carbon-tin bond and X represents an electronegative group linked to the tin atom by an atom other than carbon, together with units of at least one olefinically unsaturated comonomer.