Ferromagnetism of $^{19}\mathrm{F}$ nuclear spins in a monocrystalline ${\mathrm{KMgF}}_{3}$ sample has been observed by NMR in the presence of a high magnetic field and in the microkelvin temperature range. The three $^{19}\mathrm{F}$ spins of one crystalline cell are equivalent when the field is along the [111] axis of the crystal and inequivalent for the field along the [100] axis. At negative temperature, the structures are ferromagnetic with domains for both orientations. In the [100] orientation, it is a two-sublattice ferromagnetism: Spins belonging to different sublattices of $^{19}\mathrm{F}$ have different polarizations within the same domain.