The management objectives of Robben Island, a World Heritage Site, include the maintenance of the island's ecosystem. Yet little is known about the size and impacts of the island's population of introduced European rabbits (Oryctolagus cuniculus). We present the first quantitative assessment of the Robben Island rabbit population. Population estimates should be treated cautiously because of methodological biases, but were useful for identifying trends. Rabbit numbers fluctuated seasonally but were relatively stable between October 2003 and November 2005. Following a programme in 2006 to eradicate feral domestic cats (Felis catus), there was an apparent increase in the rabbit population and the highest estimate of rabbits was made in November 2008. By February 2009, rabbit numbers had decreased considerably and this was probably due to reduced food availability following the 2008 population explosion. Nevertheless, during 2009 rabbit numbers remained higher than they had ever been before November 2008. Rabbits usually preferred open-range habitat, but switched to transitional habitat when numbers were high. Single-species eradication programmes could have devastating impacts on the island's ecosystem. It is thus recommended that a thorough risk assessment be carried out and a holistic management strategy, rather than a single-species approach, be formulated.