ALTHOUGH gamma-ray bursts (GRBs) have been known for more than 20 years, their origin remains mysterious(1). Suggestions have ranged from nearby colliding comets(2) to merging neutron stars at cosmological distances(3). The lack of any counterpart at wavelengths other than X-rays and gamma-rays has posed a major problem in identifying the source of GRBs(4). Although in principle the distribution of energies among the burst photons, as well as their light curves, may be used to constrain the potential sources, this has proved difficult in practice(5). Here we present the observation of a particularly energetic burst with a duration of 90 minutes, which includes the detection of an 18-GeV photon. For comparison, typical GRBs emit photons in the energy range between a few keV and a few tens of MeV, and last a few tens of seconds(6,7). The extended nature of this burst holds out the hope that there will be opportunities for telescopes operating at other wavelengths to detect a GRB source white it is still active, thus providing further constraints on the source’s identity and properties.