The controlled release of an isothiazolone fungicide, C9211 (4,5‐dichloro‐2‐(n‐octyl)‐3(2H)‐isothiazolone) from a urethane oil paint is described. Leaching experiments showed that the amount of C9211 in the leachates was proportional to the loading in the paint. Paints containing 8% C9211 in the undercoat but none in the topcoat released C9211 in the same amounts as paints containing 4% C9211 in both undercoat and topcoat and the field performance of both paint combinations were identical. These results indicate that the C9211 is able to migrate through the urethane oil matrix replenishing any lost from the surface and thus giving effective antifungal control as long as biocide remains in the bulk of the paint.