X-ray methods have been used to investigate the structure of aromatic copolyimide fibers prepared from 3,3’,4,4’-biphenyltetracarboxylic dianhydride (BPDA), o-tolidine (OTOL), and p-phenylenediamine (PPD). The X-ray fiber diagrams of these copolymers contain nonperiodic layer lines that are indicative of a random comonomer sequence. The meridional peak positions are reproduced accurately in the predicted scattering for an infinite chain with random sequence and highly extended chain conformation. However, there is a less than adequate match between the observed and calculated peak profiles, which are predicted to be very much sharper than those observed. It is shown that the latter match is improved by consideration of chains of finite length with significant distortions from linearity. The results favor a sinuous chain conformation consisting of highly but not completely extended segments separated by less ordered regions or discontinuities. From analysis of the peak profiles, we have derived the correlation length in the chain axis direction, analogous to the axial crystallite size for homopolymers. This correlation length is 180 Angstrom for homopoly(BPDA-OTOL) but increases steadily with the introduction of PPD comonomer, reaching approximately 1000 Angstrom for the 60/40 BPDA-OTOL/BPDA-PPD copolymer at a x 12 draw ratio. The results are compared to those for Kevlar 149 poly(p-phenylene terephthalamide) fibers, for which the observed axial crystallite width is 395 Angstrom, much less than for the copolyimides. The correlation length and sinuosity are independent of OTOL/PPD ratio at constant draw ratio. Hence the increasing linearity obtained at higher PPD content and higher draw ratio is simply due to the presence of the more flexible BPDA-PPD units.