A laboratory beet armyworm bioassay was used to evaluate the persistence of DIMILIN® INSECT GROWTH REGULATOR (diflubenzuron) when applied at 0.062 lb ai/acre to foliage of either greenhouse- or field-grown cotton. In the first of four evaluations, plants were maintained in the greenhouse and leaves were bioassayed until 28 days after treatment (DAT). Larval survival was reduced by DIMILIN at all DAT's sampled. In the second evaluation, potted plants were maintained outside the greenhouse and leaf tissue was bioassayed until 30 DAT, during which there was a total accumulation of 2.7 inches of precipitation. Larval survival was reduced at all sampling dates by treatment with DIMILIN. In the third and fourth evaluations, field cotton was sampled for up to 34 days after application with DIMILIN during which up to 9.0 inches of rain was recorded. The bioassay again detected activity of DIMILIN throughout the exposure period. All evaluations indicated at least 4 weeks of persistence on cotton.