Since the outbreak of vector-borne West Nile virus in New York City in 1999, the disease has spread across United States and Canada, resulting in the use of larvicides such as methoprene at catch basins for widespread urban mosquito control. Although the manufacturer has recommended a methoprene dosage for catch basin application, the effect of rainfall on this dosage is not known. A field study on the fate of methoprene pellets and ingots was conducted during the summer of 2004 at three catch basins in the City of Toronto, Canada. Water samples from each catch basin were collected daily and during rain storms and analyzed for methoprene concentration using gas chromatography mass spectrometry. It was found that: (1) the methoprene concentration at the catch basin sump fell below the minimum lethal concentration most of the time; (2) rainfall events greater than 25 mm flushed methoprene pellets out of the catch basin; (3) the higher the sump water depth, the higher the residual methoprene concentration at the catch basin sump; and (4) rainfall flushed methoprene from the catch basins into the storm sewer outfall at concentrations much lower than the detrimental level which might cause ecosystem damage.