Previously reported ABF3-type ferrimagnets with the hexagonal BaTiO3 structure all contain Ni2+ as the predominant magnetic ion. This paper reports the discovery of new ferrimagnetic compositions in the RbMgF3-RbCoF3 system where Co2+ is the only magnetic ion. Phase studies on this system show that at high temperatures the hexagonal phase is stable to about 70% RbCoF3. Single crystals were grown over this compositional range and their magnetic and optical properties studied. Compositions containing in excess of about 35% RbCoF3, are ferrimagnetic and their magnetic moments (as high as 24 e.m.u. g) and Curie temperatures (up to 36°K) vary with Co2+ concentration. The magnetization data are consistent with a model of a 1 3- 2 3 ferrimagnetic spin arrangement. The B-B interaction, which is the resultant of the 90° Co2+-F-Co2+ and the direct Co2+-Co2+ interactions appears to be positive. Anisotropy measurements at 4.2°K yielded values from 2.2 × 10-6 to 8.9 × 10-6 ergs cm3 for K. Optical absorption data were used to determine the magnitude of the crystal field parameter Dq (870-880 cm-1) and the spin orbit interaction constant λ (190 cm-1). From the Dq values it is concluded that the strength of the crystalline field acting on the Co2+ ion in the hexagonal structure, where 2 3 of the CoF6 octahedra share faces, is not too different from that in cubic KCoF3 and RbCoF3, where they share corners. © 1969.