Walnut is a tree adversely affected by late spring and early autumn frosts. Therefore, it is one of the main goals to obtain late leafing and early defoliation genotypes in breeding studies on walnuts. The aim of this research is to determine the levels of phenological differences in walnut genotypes derived from the open-pollinated seeds of Bilecik, Chandler, Franquette, Howard, Kaman 1, Maraş 12, Pedro, Sütyemez 1 and Gimar (Gimnut). In this study, the bud burst, leafing, leaf yellowing and defoliation dates of seventy-five genotypes obtained from nine different walnut cultivars and their parents were determined. Cluster and PCA analyses were performed on a total of 684 genotypes to determine genetic diversity based on phenological markers. As a result of cluster analysis, all genotypes including their parents were clustered in three major groups. PCA analysis also confirmed the presence of genetic diversity-based phenological markers in our walnut seedling collection with PC1 50.226%, PC2 36.635%, PC3 12.413% and PC4 0.726% of the total variation. Pearson Correlation Coefficient among our walnut collection showed that there were very significant associations with budburst and leafing date (r = 0.97). As a result, we determined a wide variation in our walnut seedling collection, and these findings are potentially useful for future breeding studies in terms of late leafing and early defoliation.