The quaternary rare-earth phosphides REMnCu4P3 (RE = Gd-Ho) were obtained from direct reactions of the elements at 800 degrees C. They are the first examples in which ordering of two different transition-metal atoms takes place within the orthorhombic YCo5P3-type structure [Pearson symbol oP36, space group Pnma, Z = 4; a = 12.667(2)-12.6489(4) angstrom, b = 3.8119(7)-3.7755(1) angstrom, and c = 10.895(2)-10.8632(4) angstrom for RE = Gd-Ho]. Columns of trigonal prisms centered by P atoms are connected in propellor-shaped units in zigzag arrangements to generate square-pyramidal (CN5) sites that are occupied by Mn atoms and tetrahedral sites (CN4) that are occupied by Cu atoms. Spin-polarized band-structure calculations predict that the hypothetical compound YMnCu4P3 will exhibit magnetic ordering. Electrical resistivity measurements on TbMnCu4P3 indicate a poor metal.