Stoats, weasels and ferrets were officially and deliberately imported into New Zealand during the 1880s. Influential but misguided pastoralists afflicted by plagues of European rabbits were convinced that the ‘balance of nature’ had been disturbed. They persuaded the Colonial Government to see predators as potential agents of biological control capable of restoring the balance. Over the decade 1883–1892, thousands of stoats and weasels were imported from UK, and tens of thousands of ferrets were locally bred, and liberated on damaged sheep pastures. Ferrets mostly stayed there, but stoats spread into forest. Weasels lacked their staple prey, the northern voles and lemmings, so are now rare. Warnings from ornithologists of the risk to native birds were ignored, but stoats and ferrets have contributed to an ongoing massive failure in recruitment among unprotected populations of vulnerable native birds, including New Zealand’s five iconic endemic species of kiwi. Meanwhile, both rabbits and mustelids have thrived. The results of this catastrophic, legal experiment in landscape-scale biological control proved the opposite of what was expected. With funding and material supplied by pest control agencies, New Zealand has turned out to be the best place in the world to study stoats, but for the saddest of reasons.