It had been noticed in earlier work that there was a lack of close relationship between the intrinsic and contact toxicities of various compounds to adults of Anopheles stephensi List., the intrinsic toxicities being determined by topical application of solutions and the contact toxicities by exposure to dry deposits. Since the reasons probably lay in the relative rates at which compounds penetrated the cuticle from solution and from the solid state, and since these rates are likely to be differently affected by the physical properties of the compounds, the intrinsic and contact toxicities to A. stephensi of some closely related carbamates and of a miscellaneous group of organophosphorus compounds were determined, together with the solubility in n-hexane and the partition coefficient between n-hexane and water of each, and the results were compared.

No definite pattern in the relationship between these physical properties of the compounds and their intrinsic and contact toxicities emerged from consideration of the results. However, there were indications that very low solubility is a limiting factor in the uptake and penetration into mosquitos of a solid insecticide and is associated with low contact toxicity, and that, when solubility is high enough to ensure solution in the wax layer of the cuticle, contact activity is favoured by a low value for the partition coefficient between hexane and water.