A description and illustrations of the third instar of Arctodium mahdii Hawkins (Coleoptera: Scarabaeidae: Melolonthinae: Lichniini), endemic to southern Chile, are given. The description is based on larvae reared from eggs laid by adults under laboratory conditions and on larvae collected in the field. Larvae of this species feed and live in the root zones of pastures. Characteristics of this species are its separated galea and lacinia, strongly curved maxillary palpus, size and orientation of abdominal spiracles, Y-shaped anal opening, absence of palidia, and the distribution and shape of setae on the raster.