During the course of investigations into the use of the insecticides DDT and Gammexane, it has become increasingly clear that there is considerable loss through absorption into the treated surface. This loss of contact-available insecticide is most marked when simple oil solutions are used. Dosages of 100 to 200 mgms. per sq. ft., which on a non-absorbent surface (e.g. glass) are 100 per cent, toxic to flies exposed for very short periods, show little toxicity when applied to absorbent surfaces (e.g. mud).