Phosphite is a general term used to describe the salts of phosphorous acid H3PO3. It is effective in suppressing a number of plant diseases caused by oomycetes and has been shown to reduce populations of several insect species. We investigated the effects of phosphite on the Colorado potato beetles in the field and laboratory. Beetle numbers and defoliation on phosphite-treated plots were lower compared to the control plots during one out of two years of the study. No phosphite effects were detected in the field during the second year of the study. However, larval mortality was significantly higher the second year in the laboratory when larvae were fed on potato foliage excised from the potato plants treated with phosphite in the field. Laboratory tests with excised leaves dipped in a solution of phosphite revealed lower beetle survivorship and prolonged development on the treated foliage. Because of its dual properties as a fungicide and an insecticide, as well as its low toxicity to vertebrates, phosphite is a potentially good fit for integrated pest management programs.