The immature stages of the Neotropical tick Ixodes (Ixodes) pararicinus Keirans & Clifford, 1985 are described from specimens in a laboratory colony initiated from engorged females collected on cattle and larvae fed on mice and chickens. The larva and nymph of I. pararicinus are described using SEM micrographs as well as drawings for some features of the larva. Additionally, immature stages of I. pararicinus collected on wild mice and birds in Uruguay and Argentina were compared with specimens from the laboratory colony. A taxonomic key to the nymphs of the species of the ricinus complex established in the Western Hemisphere is presented. The distribution of I. pararicinus comprises Argentina, Colombia and Uruguay, but it is also probably established in Bolivia, Brazil, Chile and Peru. Most adult ticks of this species have been found on introduced domestic artiodactyles, although Neotropical deer species must have been the ancestral host. Larvae and nymphs of I. pararicinus have also been found on sigmodontine rodents and passeriform birds. Although I. pararicinus is a member of the ricinus complex, which contains the main vectors of Borrelia burgdorferi (sensu lato), there are few studies concerning its potential for pathogen transmission.