The melting and simultaneous biaxial stretching behavior of three Ziegler-Natta isotactic polypropylene resins of varying isotacticity was studied. DSC melting experiments showed that the crystallinity and melt stability of the crystalline phase depend strongly on tacticity and on the degree of undermelting, Delta T = T-m - T where T-m is the melting temperature. The level of biaxial yield stress depends on the fraction of residual "unmolten" crystallinity and the crystallite size. For specimens with same thermal history, the post-yield deformation shows strong dependence on the degree of undermelting, AT The deformation homogeneity and the resulting film thickness may be related to the thermal mobility of the chains and the tie chain and entanglement densities.