There are three ash species native to Slovenia: manna ash (Fraxinus ornus L.),narrow-leaved ash (Fraxinus angustifolia Vahl) and common ash (Fraxinus excelsior L.). Based only on morphological traits it is difficult to distinguish between closely related common and narrow-leaved ash, particularlyon sites where the two species are sympatric. Leaf and bud characteristics are mostly used as distinguishing features. However, they are highly variable and therefore in many cases unreliable. The type and size of the inflorescence are considered the best morphological traits for the separation of these two species. On the other hand, it is recommended to use as many different morphological traits as possible for determination. Incorrect identification of the two species is the main reason for inadequate knowledge about their geographic distribution in Slovenia.