We describe some flatworms (some in the genus Mesostoma) that kill mosquito larvae and may account for the variability in the population densities of Culex tarsalis and Anopheles freeborni in rice fields. When mosquito larvae brush against these worms, the larvae immediately become paralyzed and die. When C. tarsalis larvae are placed inside floating cages that exclude flatworms (50-micromter mesh), there is a fourfold increase in the their survival. Rice fields that have abundant mosquito populations lack flatworms. Most such fields have only recently been turned over to rice production, suggesting that the flatworms have difficulty dispersing to new fields but, once established, are able to overwinter and control mosquitoes for the subsequent years of rice production.