For the long-term goal of improving planting success and reforestation of floodplain forests, an underbark inoculation test was performed using one-year-old seedlings from ten different half-sib lines of narrow-leaved ash (Fraxinus angustifolia) to examine the variability of host susceptibility to the ash dieback pathogen Hymenoscyphus fraxineus. Sizes of necrotic lesions, production of epicormic shoots and mortality was recorded in all inoculated seedlings 76 days after inoculation. Mortality of different half-sib lines ranged between 13.3% and 80%. Only one of the tested half-sib lines showed necrotic lesion sizes, production of epicormic sprouts and percentage of mortality substantially lower than all the other nine half-sib-lines of the respective symptom traits. About 20% of half-sib lines had reduced only one of these three symptoms. The other 70% of tested half-sib lines had large necrotic lesions, abundant occurrence of epicormic sprouts and high percentage of mortality. Mortality was also significantly influenced by seedling height at the time of inoculation with taller seedlings developing mortality more slowly than smaller seedlings. This study demonstrates significant differences in the susceptibility to H. fraxineus between different half-sib lines of F. angustifolia and the influence of seedling height on the development of ash dieback.