The first VLBI measurement of interstellar ammonia masers is reported. The positions, as well as size and brightness temperature limits, for the masers in the NH 3 (9, 6) inversion transition that are associated with the ultracompact H II regions W51 e1 and e2 are obtained. The masers arise from two positions, each of which is located close to but not coincident with a radio continuum peak. The NH 3 masers are closer to the H 2 O masers than to the OH masers or the ultracompact H II regions. However, at least one of the two NH 3 maser spots mapped seems offset from the region of H 2 O maser activity.