The effects of methoprene S on the last instar nymphs of Ilyocoris cimicoides were investigated. Continual treatment during the last larval instar at concentrations of 0.02-0.1 ppm caused disorders in metamorphosis. Supernumerary larval instars were observed and imaginal ecdysis was frequently deranged. Morphological defects are described in detail. The compound did not affect the length of the last larval instar. It was toxic at 0.2 ppm ml(-1) of water and higher concentrations.