Aquaticus and Barbareifolius are two subgroups of Senecio aquaticus s. l. (Asteraceae) that have been recognized at various taxonomic levels in European taxonomic literature. In this paper we present the results of morphological studies on S. aquaticus s. l. in order to establish the taxonomic status of these two subgroups. Using multivariate morphometric techniques (discriminant analysis and principal component analysis), we examined the diagnostic potential of 60 macro-morphological characters and studied the delimitation of Aquaticus and Barbareifolius. Seven of the characters studied were included in the discriminant function of the stepwise discriminant analysis and are therefore considered to be the most reliable diagnostic characters of Aquaticus and Barbareifolius. Barbareifolius has, amongst other distinguishing traits, a more divaricate inflorescence than Aquaticus. Because both subgroups are distinct but overlapping clusters in multivariate morphometric space, and because clear differences in their geographical distribution are absent, we recommend the taxonomic status of variety to both groups. Following the rules of nomenclature, Aquaticus and Barbareifolius should therefore be named S. aquaticus Hill var. aquaticus and S. aquaticus Hill var. barbareifolius (Krock.) Wimm. & Grab.