Radioassay with P32-labeled Bidrin (3-hydroxy-N,N-dimethyl-cis-croton-amide dimethyl phosphate) and bioassay with cotton aphids, Aphis gossypii Glover, were used to study the systemic activity of the insecticide in cotton plants. Direct application of Bidrin to the stems of cotton plants was a more efficient method of applying the toxicant than soil injection, seed treatment, or foliar sprays.