A new species of Senecio from York, England, is described and named as Senecio eboracensis. Evidence is reviewed that this fully fertile, tetraploid species (2n = 40), which was first discovered in 1979, is a hybrid
product of S. vulgaris (2n = 40) and S. squalidus (2n = 20), and is distinct from another tetraploid hybrid product, the stabilized introgressant, S. vulgaris var. hibernicus, and also from the hexaploid hybrid product, S. cambrensis. Other studies have shown that S. eboracensis is reproductively isolated from its parents due toa high level of selfing, phenological separation, sterility of products of back crosses to S. squalidus and
reduced fertility of products of back-crosses to S. vulgaris. The morphological similarity of S. eboracensis to partially fertile, intermediate hybrid plants collected from other locations in the British Isles is discussed, and would indicate that it could arise polytopically following hybridisation between the two parent species. However, other such intermediate hybrid products do not appear to have persisted at their site of origin.