Nonanoic acid was tested under field conditions to determine its effect on mosquito larval and pupal mortality. Nonanoic acid was also applied in combination with a larvicidal petroleum oil to determine their joint action. Results showed that nonanoic acid was effective in reducing larval and pupal populations at concentrations as low as 20 ppm. In combination with the petroleum oil, nonanoic acid reduced the ovipositional activity of mosquitoes and decreased immature mosquito populations in the treated ponds.