Methoprene, a juvenile hormone analogue, has been used for at least 20 years as the primary insecticide to control the pasture mosquito Aedes nigromaculis in Fresno County, California. First reports of apparent methoprene control failures were noted in a pasture west of Fresno in September 1998. Insufficient control was noted in 12 different pastures the following season from April to September 1999. In September of 1999, field trials were conducted to better ascertain the level of control. Results based on pupal counts from different methoprene formulations and rates of application indicated that in some pastures low levels of control were achieved with Altosid (Liquid Larvicide) and Altosid XR-G. Control with Altosid Pellets was reported at 52-99%.