Mass release of parasitoids (Hymentoptera: Pteromalidae) is one possible control method of house flies (Musca domestica L.) on livestock farms. To improve the success of this method, however, there is a need for more detailed recommendations. In the present study, parasitism was evaluated in and around pens following release of the parasitoid Spalangia cameroni Perkins by hand and from containers. The study was conducted at conventional Norwegian pig farms with scattered breeding grounds for house flies. The experiment was carried out twice, with a total of seven trials of each release method followed by four weeks of monitoring parasitism by house fly sentinel pupae. No significant difference was found between the two release methods. Parasitism decreased with temperature (range 18-23°C) and was low on farms with few sites for the parasitoids to hide.