The structure of the alpha-form of poly(diphenylphosphazene) (PDPhP) has been determined and refined from powder diffraction data, using also literature electron diffraction information. The most plausible unit cell is monoclinic, space group P2(1)/b, with a = 10.23 Angstrom, b = 20.00 Angstrom, c (chain axis) = 10.18 Angstrom; gamma = 99.8(1)degrees and contains two chains with opposite side group orientation. The chain periodicity requires four monomer units, two of them non-equivalent. The standard moderately distorted (TCTC)(n) conformation of polyphosphazenes is stretched by nearly 4% with respect to the usual value, mainly widening the PNP bond angle to about 150degrees. Because only relatively weak, diffuse intensity is present on odd layer lines of fibre patterns, and due to the limited quality of the available diffraction data at higher angle, the refinement has been carried out adopting a simplified model with chain periodicity 5.09 Angstrom, i.e. assuming equivalence of all monomer units. Two over-short contacts are found, both intermolecular suggesting that the conformation with the 5.09 Angstrom repeat is acceptable for the isolated chain and that the doubling of the chain periodicity arises to optimise the packing. The proposed crystal structure represents a first simplified model of the organisation of PDPhP in the alpha-form.