The gamma-ray spectrometer (GRS) on NASA's Solar Maximum Mission satellite (SMM) has observed a significant (>5δ) net line flux at ~847 keV in the background-subtracted spectrum of SN1987A-accumulated between 1 August and 31 October 1987. This is the energy of the strong gamma-ray line from decay of 56Co, which was predicted1,3 to be seen from supernovae. The inferred average line flux during this period is ~(1.0+/-0.25) x 10-3 photons cm-2 s-1 at an energy of 843 +/- 5 keV. This feature cannot be explained by any statistical or systematic fluctuations observed in the seven previous years of GRS data. There is also evidence for the 1,238-keV line from 56Co decay, with an average flux of ~(6+/-2)xlO-4 photons cm-2 s-1 The quoted photon fluxes for both lines are preliminary. This observation confirms that 56Co is present in the supernova ejecta (as implied by the light curve4,5) and that nucleosynthesis took place during the explosion. The first appearance of the gamma-ray lines was roughly coincident with the first detections of X-rays by Ginga and MIR6,7.