The stress-strain relationship of high-density polyethylene (PE) was measured with a strain sensitivity of 2 x 10(-7) at 77 K. The internal friction was determined from hysteresis loops. It was concluded that below a strain of about 10(-3) all the nonlinear strain came from the crystalline regions by the motion of screw dislocations. Two internal friction processes were discovered which were associated with (100) [001] and(010) [001] slip. The barrier to the motion of the dislocations is pinning by the rigid amorphous material surrounding the lamellar crystals.