The present distribution of Silene maritima in the British Isles is discussed in relation to the phylogenetic history of the bladder campions and the status of the inland populations. An experimental study of morphological variation between geographically isolated populations of S. maritima is described. Large-scale divergence in a number of quantitative and qualitative vegetative and floral characters was apparent. The populations studied included coastal, arctic-alpine and inland, heavy-metal mine types, together with one mine population of S. vulgaris. Populations from mine sites proved distinct from coastal and arctic-alpine populations in their growth habit and leaf morphology. Similar, but smaller, differences in floral morphology were apparent. For every quantitative character scored, significant inter-population variation was detected. The qualitative characters scored also showed great inter-population differences. The possible evolutionary significance of morphological divergence between these S. maritima populations is discussed.