The paper presents the elements of growth and structure in a mixed culture of Euramerican poplar (Populus × euramericana (Dode) Guinier), clone Pannonia and black locust (Robinia pseudoacacia L.), at age of 10 years, established on soil type fluvisol, at a poplar trees planting distance of 5 × 5 m (400 trees per hectare) and the inter-row planted black locust at a distance of 2.5 × 1.0 m (3600 trees per hectare) on two permanent sample plots. Based on dominant and medium height of trees on sample plots it was found that they differ by 3.7 m (H100) and 4.3 m (hL), or for one site class. On both experimental plots was found approximately the same number of poplar trees per hectare, 330-343 trees, and black locust, 2760-2800 trees per hectare. On the better site classes poplar trees made larger dominant and medium breast height diameters for close to 5 cm, higher basal area by 65% and double the volume per hectare. Differences in the top and middle heights of black locust trees are less than the poplar (1.5-2.2 m), as well as differences in the middle and dominant diameters (0.6 to 1.1 cm), basal area (20%) and volume per hectare (23%). Height and diameter structure, as well as the height curve, indicating a two-story structure:, where the poplar is in the upper floor, and the black locust is in the lower floor. On a better site classes comes to intensified differentiation of trees and lagging in height growth of greater number of black locust trees in relation to the weaker site classes. The results indicate that on the weaker site classes, due to slower growth of poplar trees, establishment of the mixed culture of poplar and black locust has more justification.