We present here the synthesis, crystal structure, and magnetic and resistivity studies of a remarkable ternary reduced molybdenum oxide Pr4MO9O18 Its structure was determined on single crystal by X-ray diffraction in the space group R3m (a(h) = 11.3600(7) Angstrom, c(h) = 50.392(8) Angstrom, V-h = 5631.8(9) Angstrom(3), Z(h) = 15; a(rh) = 18.032(1) Angstrom, alpha(rh) 36.720(3)degrees). The crystal stucture contains, in addition to the common triangular Mo-3 cluster, the new Mo-7, Mo-13, and Mo-19 clusters. Contrary to the previous high-nuclearity molybdenum clusters, the Mo-13 and Mo-19 present hemispherical and spherical geometries, respectively, while the Mo-7 is planar. These four clusters with their oxygen environments form three different slabs parallel to the ab plane. The first slab is composed of Mo-3 and Mo-7 clusters in equal proportions. An infinite metallic plane based on connected Mo-3 and Mo-13 clusters forms the second slab. The Mo-13 aggregate presents a hemispherical geometry and results from the cis-edge sharing of three Mo-6 octahedra having one apex in common. The last slab is based on large spherical Mo-19 clusters that result from the hexagonal-face-sharing of two inverse Mo-13 clusters. This extensive condensation of six Mo-6 octahedra leads to an fee close-packed arrangement of the Mo atoms within the Mo-19 cluster. Over the whole structure, Mo-Mo bond lengths range from 2.520(2) to 2.860(2) Angstrom and the Mo-O bond lengths from 1.962(9) to 2.250(9) Angstrom as usually observed in reduced molybdenum oxides. Some of the rare-earths also form a slab while the remainder are located between the Mo cluster units. The Pr-O distances range from 2.215(4) to 3.102(8) Angstrom. Resistivity measurements on a single crystal show that Pr4Mo9O18 is a small band gap semiconductor and magnetic susceptibility studies are in agreement with the presence of Pr3+ ions.