We describe the diet of Grandidier's Vontsira Galidictis grandidieri (family Eupleridae), one of the rarest carnivorans in the world and restricted to an area of 400 km2 in the spiny forest of south-western Madagascar. From May 2006 to May 2007, 342 G. grandidieri scats were collected in the Parc National de Tsimanampetsotsa - 256 faecal samples in the dry season and 86 in the wet season. In total, 886 prey items were identified from these scats. Invertebrates (principally large cockroaches of the genus Gromphadorhina) were the most frequently eaten prey. Even though vertebrates comprised only 8 and 17\% of prey individuals in the diet of G. grandidieri during the dry and wet seasons, they contributed 58 and 80\% of the prey biomass in each season, respectively. About 90\% of all prey consumed had body masses below 10 g. This is well below the weight of the prey consumed by Cryptoprocta ferox, the only native carnivoran occurring in sympatry with G. grandidieri, making competition for food between the two species unlikely.