A comprehensive treatment of the small western Mediterranean genus Plagius, including morphological, anatomical and cytological details and distribution maps, is presented and the results of a phylogenetic analysis based on nrDNA internal transcribed spacer (ITS) sequence variation discussed. The three species of Plagius are found to form a monophyletic group with 88 % bootstrap support within the Leucanthemum group of genera, which is characterised by a specialised achene anatomy. The chromosome numbers of P. maghrebinus (2n = 18) and P. grandis (2n = 36) are reported for the first time. The basionym of P. grandis is lectotypified.