In an X-ray study of TSeF-TCNQ, two- and three-dimensional short-range ordering of the charge-density waves were found below ∼100 K and ∼50 K, respectively. The periodicity of the short-range order along the a-axis developing below ∼50 K is a, although that of the superlattice below 29 K is 2a From this behavior it is concluded that charge-density wave amplitudes on both kinds of stacks are comparable and that fourth-order interaction terms are important near as well as below the phase transition. Evidence is also given that it is a 2kF-CDW, not a 4kF-CDW, that is seen in TSeF-TCNQ. The differences between the roles of the fulvalene CDWs in TSeF-TCNQ and in TTF-TCNQ are discussed. © 1978.