Control of the midge, Paratanytarsus grimmii, infesting municipal water systems has proven to be difficult, because it is a parthenogenetic species that can oviposit as a pharate adult and reproduce within the system. Mean densities of P. grimmii in a midwestern USA water distribution system ranged from approximately 140 to 560 individuals/sampling date, and all 4 instars and pupae were present throughout the sampling period. Two products were tested as potential chemical controls: Cat-Floc LS, a coagulant produced by the Calgon Corporation, and 35% hydrogen peroxide, a water purifier. The results of laboratory bioassays showed that Cat-Floc LS over a 15-day period was most effective against P. grimmii.