Three females and a nymph of Ixodes stilesi were collected by dragging ground vegetation in a southern beech (Nothofagus) forest at San Martin (39° 38′S, 73° 75′W), Valdivia Province, Los Ríos Region (Region XIV), Chile, 20 April 2005 and 1 April 2006; a second nymph was removed from the long-tailed pygmy rice rat or colilargo, Oligoryzomys longicaudatus (Bennett), same locality, 20 April 2005. The 16S rDNA sequence for one of the I. stilesi females showed 100% agreement with the sequence for this species deposited in GenBank. The morphology of our nymphal specimens agrees with a recently published description of the nymph, but our females of I. stilesi differ from the redescription of a partly engorged female in four respects: 1) cornua prominent and triangular instead of small, rectangular and blunt; 2) hypostome pointed instead of rounded apically; 3) genital aperture situated between coxae III and IV instead of between coxae II and III; 4) anal groove horseshoe shaped instead of U shaped. In the female described earlier, the cornua and hypostomal apex were likely damaged when the specimen was removed from its host, while differences in the position of the genital aperture and shape of the anal groove may be due to the effects of engorgement on the integument. This is the first record of I. stilesi from O. longicaudatus.