The distribution of the Illinois state-threatened Banded Killifish Fundulus diaphanus has remained largely unchanged in Illinois from 1880 to 2000, being restricted mainly to the northeastern corner of the state. One population identified as Western Banded Killifish F. d. menona has remained stable in the glacial lakes region along the southeastern Wisconsin–northeastern Illinois border. Starting in 2001 a second population began to spread and become more common along the Lake Michigan shoreline. From there the population expanded through the Chicago Area Waterway System into the Des Plaines River and eventually the Illinois River. Historical museum specimens from this region are identified as F. d. menona, but recent specimens are identified as hybrids between F. d. menona and Eastern Banded Killifish F. d. diaphanus. Subsequently, a third population appeared in the Mississippi River near the mouth of the Rock River in 2009 and has spread from there. These individuals are also identified as F. d. menona. The rapid expansion of Banded Killifish from Lake Michigan to the Illinois River appears to be an invasion of the Eastern subspecies F. d. diaphanus and the subsequent hybridization with the native Western subspecies. It is unknown where the Banded Killifish in the Mississippi River originated, possibly from populations 160 kilometers upstream or human introductions. As the Illinois River and Mississippi River populations continue to expand their ranges, their ecological impacts are unknown at this time.