Observations of SN 1988A in M58 are presented which show a remarkable departure from Co-56 decay in the optical light curve starting about 700 days since explosion. It is shown that the departure cannot be explained by the contribution from other radioactive isotopes to the late luminosity, and fluorescence in nearby clouds can be excluded as the cause of the emission. Likely causes are a fast pulsar or interaction with circumstellar wind. Mass accretion by a neutron star appears to be a less likely explanation.