Black and white rhinoceroses are among the most charismatic megaherbivores and have become flagship species for international conservation. They are often subject to intense management that includes being compressed unnaturally in space and density. We present chromosomal and microsatellite evidence to substantiate the first recorded instance of interspecific hybridisation between them. The data suggest that the genetic integrity of the African rhinoceros species probably depends on differences in behavioural and ecological preferences that offer semipermeable reproductive isolation. We caution against the retention of both species in captive and other population situations where disruption of species-specific behaviour patterns may result if there is an unnatural composition in terms of age and sex, and where access to conspecific mates is restricted or absent.