Modification of a method used to mark Cattle Egrets (Bubulcus ibis) produced a technique suitable for color-marking Herring (Larus argentatus), Great Black-backed (L. marinus), and Laughing (L. atricilla) Gulls. Gulls marked themselves while incubating artificial eggs treated with a mixture of petroleum jelly and Rhodamine B. Marks persisted on gulls a minimum of 28-42 d. Low levels of mortality (0.05-0.27%) and nest destruction (0.46%) were associated with this technique. Substitution of Malachite Green for Rhodamine B proved ineffective, suggesting this method is best suited for water soluble dyes.