A statistical theory of nematic systems has been developed for semiflexible polymers on the basis of a new wormlike model of chain statistics. This wormlike chain model closely represents the short-range statistics of real chains and allows the evaluation of the orientation-dependent configurational partition function, which replaces the rotational partition function employed in the classical treatments of nematic systems of rodlike particles. This theory has been applied to polyethylene (PE) and polytetrafluoroethylene (PTFE) chains in the bulk, and leads to the conclusion that PE chains form a stable isotropic melt (with its first order isotropic-nematic transition expected to occur around ∼75 K), whereas PTFE forms a stable nematic phase in the melt. Furthermore, the entropy changes accompanying the isotropic-nematic transitions of semiflexible polymers are predicted to be quite small, and the predicted order parameters at the transitions are not very high, falling in the range 0.4-0.7. © 1982 American Institute of Physics.