The Filet-O-Fish was the McDonald's product introduced in 1965 that established the Filet-O-Fish name.
Franchisee Lou Groen developed the sandwich in Cincinnati, where many Catholic customers traditionally avoided meat on Fridays. The fish sandwich gave McDonald's a way to serve that customer base and helped maintain sales on a day that could otherwise be slower for a hamburger restaurant.
The product became a permanent part of McDonald's menu branding and is one of the chain's most recognizable non-hamburger items. Its name follows the company's direct, descriptive naming style: “Filet” identifies the main ingredient, while “O-Fish” gives the product a distinctive McDonald's sound.
The Filet-O-Fish is sometimes confused with later limited-time seafood products, but it has a much longer history. Its creation also shows that McDonald's branding developed through franchisee innovation as well as national advertising. A local business problem led to a product with enduring worldwide recognition.