In organic husbandry systems it is aspired that animals should have pasture access throughout the grazing season. This exposure increases the risk of infection with pasture borne parasites such as gastrointestinal nematodes (GIN). Until recently, skilful pasture management and the targeted use of anthelmintics helped to control GIN infestation satisfactorily. However, increasing resistance to all available classes of active substances makes it necessary to use anthelmintics even more sparingly so that they remain effective in the longer term. There has been increasing interest in the use of bioactive plant feeding as a sustainable gastrointestinal parasite control method for small ruminants. Plants rich in secondary metabolites, such as condensed tannins (CT), have been shown to have anthelmintic properties in vitro and in vivo. The perennial shrub heather (Calluna vulgaris) is highly abundant across European countries and is rich in CTs. Previous studies have shown that heather extracts have anthelmintic effects in vitro and in vivo. The nematophagous fungus Duddingtonia flagrans is a promising element of a complementary strategy to control GIN in grazing animals. The fungus grows naturally in the soil or in organic matter such as compost and feeds on soil nematodes, which are very similar in size and appearance to the free-living juvenile stages of GIN. Robust chlamydospores of the fungus can pass undigested through the gastrointestinal tract of grazing animals. They then germinate in the freshly deposited faeces, and trap and digest GIN larvae, which develop from eggs in parallel to the growing fungus. The aim of our study was to investigate possible antagonistic or synergistic effects of heather feeding and D. flagrans administration on the development of GIN larvae from the faeces of parasitized sheep. A feeding experiment was carried out with twelve groups of three lambs each, artificially infected with GIN. Six groups were offered fresh heather and six groups were offered a control feed with a similar nutrient content in addition to hay as a base diet for both groups. After two weeks of heather feeding, the development rate of GIN larvae was determined in quantitative faecal cultures. One animal per group was then assigned to either a normal or a low dose of D. flagrans spores or to the untreated control group. After several days of D. flagrans administration, the development rate of helminth larvae was again determined in quantitative faecal cultures.