Permethrin has been recommended for use in predator exclusion studies ofAmblyseius limonicusGarman&McGregor (formerly inTyphlodromalus), an important predator of tetranychid cassava (Manihot esculentaCrantz) pests: green mite,Mononychellus progresivusDoreste, and twospotted spider mite,Tetranychus urticaeKoch. Among criteria for a chemical exclusion agent are selective toxicity and lack of side effects such as enhanced pest fecundity, stimluation of plant growth, or improvement of host plant nutritional quality. Laboratory and field tests provided no evidence for phytostimulation, change in cassava nutritional quality, or increased spider mite fecundity; however, abundance of nontarget arthropods was significantly reduced in permethrin-treated field plots. A possible repellency effect of permethrin onM. progresivuswas suggested under laboratory conditions. Implications of repellency and lowered abundance of nontarget species for field use of permethrin as a predator exclusion agent are discussed.