Na2Mo4O13 · 6H2O obtainable by the acidification of sodium molybdate solutions crystallizes as needles which are constituted of tiny fibers. Under mild hydrothermal conditions crystal needles can be obtained in which the fibers exhibit a certain order so that a rough structure determination could be carried out by the single crystal method and the molybdenum positions ascertained. The compound crystallizes in the space group P21 with the lattice constants a = 21,202 Å, b = 10,160 Å, c = 7,81 Å, β = 100,1°. It contains chain-like anions which are evidently made up from Mo8O28 units. The concatenation of there moieties is a novel feature