The Iowa darter (Etheostoma exile) was first captured in Little Yampa Canyon, Yampa River, Colorado, by electrofishing in autumn 2003, and abundance of this nonnative species increased during 2004–2007. The Iowa darter also expanded downstream 229 river km, based on captures of young fish in a drift net at the confluence of the Yampa and Green rivers in 2005, and in light traps in the Green River near Jensen, Utah, in 2005–2007. Likely introduction of the Iowa darter via bait-bucket transfer suggests that additional legal deterrents might be needed to reduce further illicit introductions of potentially problematic nonnative fishes in the Colorado River Basin.