The structure-activity relationships of insecticidal 3-substituted-α-cyanobenzyl 3-(2, 2-dichlorovinyl)-2, 2-dimethylcyclopropanecarboxylates have been analyzed by the Hansch method against housefly, Musca domestica. The variation of the activity was shown to be mainly related to the variation of the hydrophobicity of the 3-substituent. The activity of the compounds whose 3-substituent has either π electrons or a lone pair at the β-position was about 2 times higher than the other compounds.