McDonald’s first major fish sandwich, introduced nationally in 1965, was the Filet-O-Fish.
McDonald’s franchisee Lou Groen developed the sandwich in Cincinnati, Ohio, where many customers observed Catholic Friday abstinence from meat. The fish sandwich provided a menu alternative during a period when hamburger sales could fall on Fridays.
McDonald’s tested the Filet-O-Fish before adding it nationally in 1965. The original sandwich used a fried fish fillet, tartar sauce, and cheese on a bun. Its continuing presence helped make seafood part of McDonald’s recognizable product branding alongside burgers and fries.
The Filet-O-Fish is sometimes confused with later limited-time seafood products. Its significance is that it became a permanent, nationally marketed McDonald’s sandwich rather than a short promotional item.