Ternary rare-earth zinc arsenides REZn3As3 (RE = La-Nd, Sm) with polymorphic modifications different from the previously known defect CaAl2Si2-type forms, and the corresponding rare-earth cadmium arsenides RECd3As3 (RE = La-Pr), have been prepared by reaction of the elements at 800 degrees C. LaZn3As3 adopts a new orthorhombic structure type (Pearson symbol oP28, space group Pnma, Z = 4, a = 12.5935(8) angstrom, b = 4.1054(3) angstrom, c = 11.5968(7) A) in which ZnAs4 tetrahedra share edges to form ribbons that are fragments of other layered arsenide structures; these ribbons are then interconnected in a three-dimensional framework with large channels aligned parallel to the b direction that are occupied by La3+ cations. All remaining compounds adopt the hexagonal ScAl3C3-type structure (Pearson symbol hP14, space group P6(3)/mmc, Z = 2; a = 4.1772(7)-4.1501(2) angstrom, c = 20.477(3)-20.357(1) angstrom for REZn3As3 (RE = Ce, Pr, Nd, Sm); a = 4.4190(3)-4.3923(2) angstrom, c = 21.4407(13)-21.3004(8) angstrom for RECd3As3 (RE = La-Pr)) in which [M3As3](3-) layers (M = Zn, Cd), formed by a triple stacking of nets of close-packed As atoms with M atoms occupying tetrahedral and trigonal planar sites, are separated by La3+ cations. Electrical resistivity measurements and band structure calculations revealed that orthorhombic LaZn3As3 is a narrow band gap semiconductor.