Several materials have been identified over the past few years as promising candidates for the development of new generations of magnetoresistive devices. These range from artificially engineered magnetic multilayers’ and granular alloys(2,3), in which the magnetic-field response of interfacial spins modulates electron transport to give rise to ’giant’ magnetoresistance(4), to the manganite peravskites5-7, in which metal-insulator transitions driven by a magnetic field give rise to a ’colossal’ magnetoresistive response (albeit at very high fields). Here we describe a hitherto unexplored class of magnetoresistive compounds, the silver chalcogenides. At high temperatures, the compounds Ag2S, Ag2Se and Ag2Te are superionic conductors; below similar to 400 K, ion migration is effectively frozen and the compounds are non-magnetic semiconductors(8,9) that exhibit no appreciable magnetoresistance(10). We show that slightly altering the stoichiometry can lead to a marked increase in the magnetic response. At room temperature and in a magnetic field of similar to 55 kOe, Ag2+deltaSe and Ag2+deltaTe show resistance increases of up to 200%, which are comparable with the colossal-magnetoresistance materials. Moreover, the resistance of our most responsive samples exhibits an unusual linear dependence on magnetic field, indicating both a potentially useful response down to fields of practical importance and a peculiarly long length scale associated with the underlying mechanism.