The onset of sharkskin and wavy extrudate distortions was studied in a linear low-density polyethylene which was diluted up to 40 wt.% with octadecane in order to vary the plateau modulus. The ratio of critical wall stress to plateau modulus remained constant for the onset of sharkskin. The transition to wavy flow occurred at a constant wall stress, independent of plateau modulus for dilutions up to 30 wt.%, after which the discontinuity in the flow curve and the transition to the wavy instability did not occur. The different behavior for the two transitions suggests different mechanisms.