Ixodes dammini Spielman, Clifford, Piesman & Corwin was found for the first time in Illinois in November 1987, when two adult females were collected from two deer in Jo Daviess County in the northwestern corner of the state. In 1988, in a study of six state parks in northern Illinois, questing adults and nymphs were encountered in one park in Ogle County. During the firearm deer hunt in November 1988, adult female and male ticks were found in several counties, with a high rate of infestation (greater than 25%) in two counties (Ogle and Rock Island) along the Rock River, which flows from Wisconsin into the Mississippi River. Several cases in humans with no history of travel outside of the state have been reported, primarily from northern Illinois. We suspect that infiltration of infected ticks and wildlife from Wisconsin is resulting in the emergence of Lyme disease in Illinois. Because all the components necessary for the completion of the tick life cycle and for the transmission of Borrelia burgdorferi are available throughout much of the state, I. dammini and Lyme disease can spread and become established in large portions of Illinois.