A new manganese gallium phosphate, Mn-3(H2O)(6)Ga-4(PO4)(6), has been synthesized under hydrothermal conditions at 150 degrees C and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectroscopy. It crystallized in the monoclinic space group, P2(1)/n, with a = 8.9468(4) Angstrom, b = 10.1481(5) Angstrom, c = 13.5540(7) Angstrom, beta = 108.249(1)degrees, and Z = 2. The compound is unusual in that it is not only the first nonorganically templated MnGaPO phase but also the first instance where edge-shared trinuclear manganese-oxygen clusters are encapsulated in a metal phosphate lattice. The trimer involves a central Mn(H2O)(4)O-2 octahedron, which links to two Mn (H2O)(2)O-4 octahedra at trans edges. The Mn-3(H2O)(6)O-8 clusters reside in tunnels built from GaO5 trigonal bipyramids and PO4 tetrahedra. Our magnetic study revealed that superexchange interactions occurred between the neighboring Mn-II centers. A good fit of the magnetic susceptibility data for the isolated trimers was obtained by using a derived expression based on Van Vleck's equation. Unlike all existing linear trinuclear Mn-II complexes, the chi(M)T product in the range 8-4 K remains at a constant value corresponding to one spin S = 5/2 per three Mn-II centers. The Curie behavior at such low temperatures has been confirmed by EPR data. According to the thermogravimetric analysis/differential thermal analysis (TGA/ DTA) results, the title compound is thermally stable up to ca. 200 degrees C.