For a series of semicrystalline ethylene-alpha-olefin copolymers (linear low-density polyethylene, LLDPE) produced by metallocene and Ziegler-Natta polymerization, the effect of the copolymer composition on the morphology and the evolution of linear viscoelastic properties during isothermal crystallization was studied. The put-pose of this study is to explore the extreme sensitivity of rheology and processing to small variations in molecular detail in LLDPE. The metallocene LLDPE sample contains fewer branched units (2.6 mol % C-6) than the Ziegler-Natta copolymer (3.8 mol % C-6), but still, it melts at lower temperatures than the more branched Ziegler-Natta LLDPE analogue. Physical gelation as observed by rheology occurs over a broader temperature region for the metallocene LLDPE than for the Ziegler-Natta copolymer. DSC and rheological observations indicate that the solidification behavior in LLDPEs is determined more by the composition distribution than by the overall content of branched units. A HDPE was included in the study for comparison purposes.