The distribution of the Iowan Brood of periodical cicadas in Illinois was determined in 1997 using a global positioning system receiver and mapping software. The area contained within the distribution derived from this survey was approximately twice the area bounded by a range map published in 1975. Periodical cicadas were found in many areas not included in the earlier range map, and were absent from several areas that had been included. Periodical cicadas were patchily distributed within the boundary, being most abundant in stream woodlands and protected areas and least abundant in agricultural and urban areas. Periodical cicadas were primarily found in a soil type associated with stream woodlands, and appeared to follow forests along waterways, but it is possible that these areas serve as refugia from human activity rather than preferred habitats.