The organophosphorus insecticide, azamethiphos, is widely usedthroughout the world to control the housefly, Muscadomestica (L.). Since its commercial introduction to Denmark in1983 for this purpose, we have monitored the toxicity of azamethiphosto housefly populations at livestock farms throughout the country andcarried out regular field studies. The findings of our field studies,which have revealed a strong potential for resistance development, havebeen born out by regular surveys showing that resistance has increasedin recent years. Through the analysis of a field derived laboratorystrain, we have implicated oxidative and hydrolytic mechanisms togetherwith altered acetylcholinesterase in this resistance. Our field andlaboratory studies have also indicated that resistance is relativelyunstable, and can revert in the absence of selection. The implicationsof our findings for the continued efficacy of azamethiphos arediscussed.