Technical Abstract: The growing interest in using open-pollinated varieties (OPVs) and varietal hybrids (OPVhs) of corn (Zea mays L.) especially in breeding programs for organic and low-input farming reflects the value of large plasticity levels available in their plant, ear, and kernel traits. We estimated and partitioned total phenotypic variance into its components, quantified population structure and estimated phenotypic diversity and broad sense heritability (H2) for 35 traits measured on the 46 OPVhs grown under four environments (E). A large portion (68%) of total variation in all phenotypic and yield traits of the 46 OPVhs was partitioned among heterotic groups of their maternal parents (HGs), and was associated with large levels of mean population differentiation (GST; 0.53-0.80) of traits across HGs. Significant levels of variation, whether based on a matrix of phenotypic traits (P1) or grain yield (P2), were found among HGs and among OPVhs(HGs). Most traits were significantly correlated (r=|0.22-0.83|) with grain yield per plant (GY); however, a few traits displayed significant OPVhs(HGs) x E interaction. Average GY across environments was inversely related to its variance; nevertheless, we identified OPVhs with large GY associated with small variance along with the traits contributing to this large and stable GY. Estimates of H2 ranged from 0.35 to 0.85, with decreasing average values for kernel (0.69), ear (0.62), and plant (0.53) traits and a concomitant increasing level of OPVhs(HGs) x E interaction. Pairwise phenotypic distances among OPVhs indicate that most OPVhs(HGs) have different P matrices; and, therefore, will have different breeding potential.