Extensive stands of Cerastium dubium (Caryophyllaceae) have been located in two contiguous counties (Effingham and Fayette) of central Illinois. This is only the second report of occurrence in North America and the first for the eastern half of the continent. A key is presented, in which this weedy species is compared with and distinguished from seven sympatric members of Cerastium growing in fields or ruderal habitats. Chromosome counts of five species are included, which support previous reports of a basic chromosome number of x = 18 (or 9?) for the genus. In contrast to the published meiotic number of n = 19 for European populations of C. dubium, we obtained n = 18 for the Illinois populations.