We report on X-ray diffraction and grazing-incidence X-ray diffraction data of poly(9,9-dinonylfluorene) (PF9) in bulk, thin films and in the 1% methylcyclohexane gel. We denote the main crystalline phase as alpha phase and propose that the unit cell is monoclinic (a = 29.31 angstrom, b = 23.65 angstrom, c = 33.33 angstrom, and gamma = 84.70 degrees) in bulk and orthorhombic (a = 28.70 angstrom, b = 23.48 angstrom, and c = 33.23 angstrom) in thin films. This structure corresponds to the layered structure along the a-axis (along the elongated side chains and perpendicular to the seemingly stiff polymer chains) and to the stacking of aromatic main chain units along the b-axis. The polymer chains are aligned along the c-axis. Monoclinic structure agrees with the layer spacing of 14.6 angstrom, the stacking period d(040) = 5.89 angstrom and the monomer repeat distance of 8.33 angstrom. The alpha phase experiences an order-disorder transition at 170 degrees C upon heating. In the 1% methylcydohexane gel, this structural motif is maintained but with the loss of long-range order. This is interpreted as a formation of mesomorphic beta phase with an orthorhombic unit cell (a = 29.1 angstrom, b = 28.1 angstrom, and c = 16.7 angstrom). Structural analogues to other 9,9-di-n-alkyl-substituted polyfluorenes are discussed in terms of unit cell parameters and backbone geometry.