A double-resonance nuclear magnetic resonance, NMR, signal between the very mobile nuclei of a superionic conductor has been detected for the first time in a silver conductor. The heteronuclear double-resonance signal between mobile nuclei should not be observed in the fast motion limit if the inter-cationic interactions are completely removed. However, a Ag-109-Ag-107 double-resonance signal has in fact been observed in a superionic conductor of 0.1-1 S m(-1), namely Ag9GaSe6. This implies that the cationic motion is correlated in this solid electrolyte and some residual interactions, perhaps including chemical bonds among very mobile cations, may remain even in the fast motion limit.