Cox and Box; or, The Long-Lost Brothers, is a one-act comic opera with a libretto by F. C. Burnand and music by Arthur Sullivan, based on the 1847 farce Box and Cox by John Maddison Morton. It was Sullivan's first successful comic opera. The story concerns a landlord who lets a room to two lodgers, one who works at night and one who works during the day. When one of them has the day off, they meet each other in the room and tempers flare. Sullivan wrote this piece five years before his first opera with W. S. Gilbert, Thespis.
The piece premiered in 1866 and was seen a few times at charity benefits in 1867. Once given a professional production in 1869, it became popular, running for 264 performances and enjoying many revivals and further charity performances. During the 20th century, it was frequently played by the D'Oyly Carte Opera Company in an abridged version, as a curtain raiser for the shorter Gilbert and Sullivan operas. It has been played by numerous professional and amateur companies throughout the world and continues to be frequently produced.