Isotactic poly(4-methylpentene-1) melt-spun fibers were investigated. Prior investigators of melt-spun fibers found that these fibers have a tetragonal unit cell (Form 1). We obtained the same unit cell structure in melt-spun fibers. We found that higher draw-down-ratio fibers had d-spacings closer to the previously cited values of Form I. We also found that cold-drawn fibers had similar values to those of melt-spun fibers. However, after these were annealed at 200 degrees C, the unit cell was changed. It is possible that this new unit cell was the orthorhombic form of He and Porter. We also observed the birefringence of these fibers. The values changed after the melt-spun fibers were cold drawn and annealed. The melt-spun fiber values reached 0.006. The values for the drawn fibers were as high as 0.007. We suggest that the intrinsic birefringence is about 0.0075. (c) 2005 Wiley Periodicals, Inc.