Gas-liquid chromatography (electron-affinity) was used to study the fate of dieldrin in stable flies, Stomoxys calcitrans (L.), highly resistant or susceptible to this compound. Results showed no evidence that metabolism was enhanced or that excretion of dieldrin was more rapid in resistant flies than in susceptible flies. There was some indication that penetration was slower in resistant females than in any of the other flies, but this result may have been associated with the more concentrated solutions with which these flies were treated.