The fish sandwich introduced by franchisee Lou Groen was called the Filet-O-Fish.
Groen operated a McDonald's restaurant in Cincinnati, Ohio, where many nearby customers observed meatless Catholic Fridays. To attract those customers, he developed a fish sandwich with a breaded fish fillet, cheese, and tartar sauce on a bun.
McDonald's tested the product in 1962, and the Filet-O-Fish became a permanent menu item after performing strongly against a competing ham-based sandwich proposed by Ray Kroc. The product gave McDonald's a branded menu option for customers who did not want beef.
The name is sometimes written informally as “Filet of Fish,” but McDonald's official product styling is “Filet-O-Fish.” Its hyphenated name became part of the product's distinct brand identity.