The house fly parasitoid Muscidifurax raptor Girault & Sanders was isolated from the pigger dung heaps of an agricultural cooperative in the district Potsdam, GDR. In 1979 and 1980 a maximum parazitation rate of the housefly population of more than 50% was found. In June M. raptor starts to increase and continued to be fairly abundant to the end of the year. In winter and spring the abundance of the parasitoid was very low. Releases of 90,000 M. raptor led to a parazitation of the fly pupae somewhat earlier than normal but with no recognizable suppression of Musca domestica, which resulted from the unsufficient number of released parasitoids considering the existing population of the house fly.