INTERGALACTIC magnetic fields (IGMFs) can be produced by a number of mechanisms, but are expected to be weak and have not so far been detected, ’Primordial’ magnetic fields might have been produced in the very early Universe, either by quantum fluctuations during the ’inflationary’ period(1,2) or through the decoupling transitions of the fundamental forces(3,4). The much later ejection of magnetized plasma into intergalactic space from galaxies and active galactic nuclei should also produce IGMFs, though it is possible that some fraction of the Universe retains its ’primordial’ field(5). Previous studies(6) have placed an upper limit of 10(-9) gauss on the strength of an IGMF (with a coherence length of 1 Mpc), but the strength may be much less than this, posing a formidable challenge to current observational capabilities. Here I propose a highly sensitive method for probing weak IGMFs by exploiting their effect on the arrival times of gamma-rays from extragalactic sources. The delay in arrival owing to the action of intergalactic magnetic fields on electron cascades caused by scattering of the gamma-ray photons might be used to measure fields as weak as 10(-24) gauss. I suggest that this effect may already have been seen in the arrival times of high-energy photons after the main burst of a gamma-ray burster(7).