The taxonomic delimitation of Hesperis bicuspidata (Willd.) Poir. and H. stellata F. Dvořák is problematic, and we therefore studied morphological, palynological, seed-coat surface and karyological features of both species to determine their degree of similarity. The morphology was described using 13 characters, each based on ten measurements on different specimens. Hesperis stellata and H. bicuspidata both show large variation in vegetative and reproductive organs; petal length varies between 18 and 26 mm and petal width is 3-7 mm; the corolla is lilac, whitish lilac, violet, dark violet or white in color, and fruits of both species are generally glabrous or have very short bifurcate hairs. Pollen and seed morphology were examined by LM and SEM. Pollen of both species is prolate-spheroid or subprolate, tricolpate and reticulate. Their exine structures are the same. Hesperis bicuspidata is diploid (2n=14) but different chromosome types were present in the two studied specimens. Hesperis stellata is tetraploid (2n=28) showing the same chromosome types as the samples of H. bicuspidata. Eighteen morphometrical characters of H. bicuspidata, H. stellata, H. pisidica and H. armena were analysed by ANOVA. Notes on ecology and distribution are also presented for H. bicuspidata. Based on the results, H. stellata is reduced to a synonym of H. bicuspidata. © 2011 The Authors. Nordic Journal of Botany © 2011 Nordic Society Oikos.