The duck-billed platypus (Ornithorhynchus anatinus) is a unique egg-laying mammal, with lactation, venom and a bill. It even has an electro-sensory system for foraging underwater. Platypuses are monotremes descended from the most basal branch of the mammalian lineage and combine aspects of both reptilian and mammalian biology. Now an international consortium reports the sequence and analysis of the platypus genome. It is an amalgam of reptilian, mammalian and its own unique characteristics that provides clues to the function and evolution of all mammalian genomes. This has helped uncover the origins of genomic imprinting; platypus venom proteins were co-opted independently from the same gene families that provided reptile venom; milk protein genes are conserved; and immune gene family expansions are directly related to platypus biology. As well as providing an invaluable resource for comparative genomics, the sequence will be important for monotreme conservation. The cover image (by D. Parer & E. Parer-Cook) shows the unique bill with electro-sensory pits, eye and ear opening behind the eye.