A component of the event-related magnetic field (ERMF) response was observed in magnetoencephalographic signals recorded during the maintenance of information in visual short-term memory (VSTM). This sustained posterior contralateral magnetic (SPCM) field is likely the magnetic equivalent of the sustained posterior contralateral negativity (SPCN) found in electrophysiology. Magnetoencephalography data showed, at the sensor level, a bilateral activation over the parietal cortex that increased in amplitude for higher memory load. Others sensors, also over the parietal cortex, showed an activation pattern similar to the SPCN with higher activation for the hemisphere contralateral to the visual field from which visual information was encoded. These two activation patterns suggest that the SPCN and SPCM are generated by a network of cortical sources that includes bilateral parietal loci, likely intra-parietal/intra-occipital cortex, and contralateral parietal sources.