House flies are important vectors of many human and animal diseases. For this reason insecticides have been used to control them, but resistance will often develop. Spinosad is an insecticide relatively new and promising for house fly control. In the laboratory it was shown, however, that 150-fold resistance could develop after 10 generations of selection. Because of this we decided to monitor the development of resistance under field conditions. Six different field strains of house fly were collected during the summer of 2004 from various hog, dairy and poultry facilities. Spinosad had never been used at these facilities, although other insecticides were used. These flies were topically assayed with spinosad insecticide to determine base line data for field resistance. During the beginning of summer 2005, house flies were collected from eight different California and New York dairy farms where spinosad had never been used but was scheduled for use that season. These flies were topically assayed and labeled "preseason" strains. At the end of the spraying season, after spinosad had been used, house flies were collected again, labeled "postseason" strains, and topically assayed. No detectable development of resistance was found over one spraying season.