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South Africa is rich in biological diversity, but measures to conserve this heritage are under-funded and are of relatively low priority at national level. Part of the problem is that the social value of biodiversity is unknown, and thus the potential impact of a loss of biodiversity on social wellbeing is not recognised. Some of these threats, particularly climate change, are predicted to have major impacts on biodiversity within the next 50 years. This study investigates the public interest, experience and knowledge of biodiversity and uses contingent valuation methods to estimate its existence value, with emphasis on the internationally significant fynbos biome in the Western Cape. More than half of respondents classified themselves as actively or passionately interested in nature, and a high proportion had recently visited major nature reserves. Interest was correlated with knowledge, and both were positively correlated with willingness to pay (WTP) for biodiversity conservation, though WTP was constrained by income level. WTP for conservation was relatively high ($3.3 million per year for fynbos, $58 million for national biodiversity), and comparable with government conservation budgets. WTP increased dramatically (to up to $15 million and $263 million per year, respectively) when respondents were faced with the predicted impacts of climate change on biodiversity. The latter is probably a better estimate of the full existence value of biodiversity, since respondents were faced with absolute losses rather than, say, the reduction in species diversity.