GALAXIES at high redshift are very faint and difficult to study at optical and near-infrared wavelengths, but detection of far-infrared emission(1) and molecular gas(2,3) in a galaxy at redshift z approximate to 2.3 has suggested that their early evolution may be investigated by these means instead. The host galaxies of quasars are promising candidates for these observations, particularly as quasars might be triggered by interactions and mergers between galaxies(4,5) which result in dust- and gas-rich systems. The Cloverleaf, a gravitationally lensed quasar, has far-infrared/submillimetre emission indicating a substantial dust content(6), and therefore potentially a large amount of gas. Here we report the detection of carbon monoxide emission from the Cloverleaf, which we interpret as indicating a mass of molecular gas that is comparable to the total dynamical mass of the host galaxy, and which is consistent with the total baryonic content of a present-day luminous galaxy. This suggests that, although some processing of gas through stars has taken place in the Cloverleaf at a lookback time of 85% of the current age of the Universe, much of the future stellar content has yet to be formed.