A general ecological study was made of the Northern viper (adder) in Dorset. The principal aim was to obtain a detailed account of its seasonal activity and then to relate this to its habitat and other requirements. 166 marked individuals were studied in the field for at least three seasons and additional information was obtained from 194 post-mortem examinations and from studies of live individuals in an enclosure. It was found the viper occupies two distinct habitats, high, dry ground for winter hibernation and low-lying damp river meadows in the summer. Feeding essentially occurs from June to August and breeding is restricted by the short active season to alternate years. This study forms part of the research being carried out to help with the conservation of the British reptiles on the National Nature Reserves in this part of England.