Conventional control of two of New Zealand's principal vertebrate pests, the brushtail possum Trichosurus vulpecula and the European rabbit Oryctolagus cuniculus may not be sustainable on economic or social grounds. Biological control through the deliberate release of debilitating pathogens or vectored fertility control offers a potentially long-term, target-specific and humane option. However, the release of exotic pathogens, possibly genetically modified, into New Zealand raises major social and political concerns on both national and international levels which need to be weighed carefully against economic and conservation gains.