none In the host-parasite-system Spodoptera littoralis/Microplitis rufiventris the oral administration of Dimilin (Diflubenzuron) affects differently the larval instars of the host and the endoparasite. The results obtained reveal that the non-parasitized Spodoptera larvae (L sub(4)/1. day) were more susceptible to Dimilin than those which had been parasitized. Additionally there was no evidence in either group of a linear relation between dose and mortality. Dimilin affected Microplitis) by the following ways: larvae did not emerge from the host (0-14.1%) or incompletely emerged (0-43.4%), the larvae left the host but failed to build a complete cocoon (2.2-13.3%) or died during the cocoon stage (0-14.4%). These anomalies included encapsulation accompanied by melanization of the cuticle of the parasite and the death of the Microplitis larva. The peritrophic membrane and the mid gut were severely damaged. The mortality of the parasites was related to the time of Dimilin application. A high susceptibility of the second stage of Microplitis was indicated by higher rates of mortality, when treatment occurred at this stage.