Micro-beam electron diffraction has been used to examine lath-shaped precipitates that form in vacancy-depleted regions adjacent to grain boundaries in the alloys Al-4Zn-3Mg-(0.1, 0.2, 0.3 Ag) wt. %. These laths, which have a {111}(alpha). habit plane, have been identified as the eta(4) orientation of the equilibrium phase eta (MgZn2). Selected area electron diffraction patterns recorded from the eta(4) laths were essentially identical to patterns recorded from the fine eta' precipitates. The laths appeared to nucleate uniformly in vacancy-depleted regions, as confirmed by direct quenching investigations. Contrary to previous suggestions, reversion experiments revealed that G.P. zones were not essential to nucleate fine dispersions of eta' throughout the grains during ageing at elevated temperatures. On the basis of these results, it appears that additions of Ag to Al-Zn-Mg alloys may stimulate the nucleation of both eta' and eta(4) precipitates.