Traditional farming practice in Denmark covers small animal units of 30-70 lactating cows that commonly are mixed with 10-20 sows for the production of feeder swine. Nowadays this picture has changed towards few but large farming units of several hundred lactating cows or swine farms where thousand of individuals yearly are produced for the export market. Whatever the old traditional farming system or the modern ones the house fly (Musca domestica) and the stable fly (Stomoxys calcitrans) are still the two most common and annoying pest species in Danish confined livestock production. Over the past 30 years pteromalid pupal parasitoids have been studied intensively in Denmark with respect to natural occurrence, species, relative abundance, parasitism, dispersal and their effect in suppressing the fly populations below nuisance level. This presentation will give an overview of the knowledge gained on the pupal parasitoids and Spalangia cameroni in particular in Denmark. Furthermore, knowledge on the black dump fly (Hydrotaea aenescens), which coincidently was found on a Danish farm about 25 years ago, and since has been widely used to control house flies in modern swine production units, will be presented as well. Especially, some recent results based on release studies with the emphasis to measure the suppressive effect on larvae of house flies and stable flies in straw embedded areas of dairy cattle farms.