Insecticides azinphos ethyl, endosulfan, acephate, methamidophos, and ENT 27041 were tested in the laboratory against various stadia ofPhthorimaea operculella. Azinphos ethyl was superior as an ovicide and larvicide but was a poor adulticide. Endosulfan and acephate were good adulticides. All were effective against larvae inside foliage mines. Insecticide deposits on leaves had repellant activity against ovipositing females, greatest activity being shown by ENT 27041. No apparent resistance to the commonly used insecticides azinphos ethyl and endosulfan was detected in the field population. A comparison of azinphos ethyl and azinphos methyl showed azinphos ethyl to be slightly superior. Acephate and methamidophos exhibited strong systemic activity when applied to the soil but were systemically ineffective when applied to the foliage. Desirable properties of an insecticide for use against potato tuber moth include strong ovicidal and larvicidal activity and reasonable longevity. These criteria were best satisfied hy azinphos ethyl.