The preceding production of essential numbers of hosts--puparia of Musca domestica L.--requires more labour and space and higher costs than the virtual rearing of the hymenopterous parasitoid Muscidifurax raptor GIRAULT & SANDERS itself. Therefore, the possibility of preserving the host by freezing would be a substantial improvement in the mass rearing of the parasitoid. It was demonstrated that frozen puparia were not less attractive for females of M. raptor than fresh ones. Puparia less than 24 hours old are not suitable for cold storing. Host pupae stored under freezing temperatures or heat (treated for 5 min at 65 degrees C) showed a higher hatching rate of the parasitoid than fresh ones. Freezing temperatures of about -7 degrees C are not sufficient for longer storage. At a temperature of -21 degrees C the host suitability of the frozen puparia did not decrease for a storage period of 53 weeks. The average size of M. raptor remained unchanged after rearing 8-9 generations upon coldstored hosts.