The compound NaMgF3 occurs in the system NaF-MgF2 and forms a eutectic with either component. The eutectic between NaF and NaMgF3 has been unidirectionally solidified and the phase morphology and crystallography examined by optical and electron microscopy coupled with electron diffraction. Solidification with the “complex-regular” morphology from faceted cellular projections on the solid/liquid interface occurs for compositions on both sides of the eutectic point. Extensive regions of rod-like morphology occur in specimens where one of the macrofacet planes, constituting the faceted projections, is perpendicular to the direction of heat extraction. An analysis of twinning observed in the NaMgF3 phase shows that it occurs in accommodation of lattice strain associated with a distortion of the unit cell axes on cooling from the growth temperature. These small distortions involve a progressive reduction in crystal symmetry from a cubic high temperature form to orthorhombic at ambient temperatures. The twins are not associated with the eutectic growth process in providing re-entrant edges to facilitate coupled growth of the two phases.