Implications of phenotypic plasticity in a subspecific numerical taxonomic study ofOrnithogalum montanumCyr. (Liliaceae) are discussed. Clones belonging to six natural populations were grown in a glasshouse, and their morphological response to three water dosages was analyzed by means of principal component analysis. PC-1 ranks the three groups of replicated populations, suggesting a high degree of phenotypic plasticity; on the other hand, PC-3 is almost environmentally independent. Proximities in the phenetic space are shown to be at least partially environmentally dependent, suggesting a reaction norm for the character correlation matrix. The results do not corroborate a previous recognition of six subspecies ofO.montanum.Key words: phenotypic plasticity, numerical taxonomy,Ornithogalum, reaction norm, principal component analysis.