Since the discovery of superconductivity in the high-transition-temperature (high-Tc) copper oxides two decades ago, it has been firmly established that the CuO2 plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO1-xFx has recently been discovered1, 2, 3, 4, 5, 6, 7, 8 and the conspicuous lack of the CuO2 planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO0.85F0.15 with Tc = 42 K as measured by Andreev spectroscopy. The gap value of 2 = 13.34 0.3 meV gives 2/kBTc = 3.68 (where kB is the Boltzmann constant), close to the Bardeen–Cooper–Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at Tc in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-Tc copper oxides.