The significance of the well-known central roundel of the mosaic from Hinton St Mary, Dorset, UK, which carries a young male head with a chi-rho monogram behind it and pomegranates on each side, has been much discussed. This paper marshals evidence which suggests that the head is not a representation of Christ, but of one of the emperors of the House of Constantine, perhaps Constantine himself. Some of the implications for the nature of fourth-century imperial culture are discussed.