The transformation matrix method and the concept of virtual bonds has been applied to the study of the persistence lengths of two recently developed polyimide fibers, PMDA-PFMB, synthesized from pyromellitic dianhydride (PMDA) a n d 2,2'bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB), and BPDA-PFMB, synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and PFMB. The elements of the averaged transformation matrices are obtained from a molecular dynamics trajectory of 1 ns and from a consideration of the symmetry of the torsion potential energy function for selected bonds. The persistence length is found to be -496 8, for PMDA-PFMB and -66 8, for BPDA-PFMB. The large persistence length of PMDA-PFMB, comparable to that of the rodlike polymer poly(benzobisoxazo1e) (PBO), is caused by the colinearity of the virtual bonds in t h e chain and the stiffness of the bond angles. The inclusion of the electrostatic interactions does not seem to affect the result significantly, although a very slight decrease (-5%) of the persistence length was detected. The effect of substitution of methyl groups for the trifluoromethyl groups in these polyimide fibers is also studied, with a n increase of -14% in the persistence length of PMDA-PFMB and -1.4% in the persistence length of BPDA-PFMB. Since a good solvent that would be appropriate for use in a light scattering measurement has not been found for either of these polyimide fibers, the theoretical estimates of the persistence length cannot be compared with experiment.