Coprophagous insect communities play a critical role in the decomposition of vertebrate dung and provide ecosystem functions fundamental to modern agriculture. While the ecology of dung beetles is rather well understood, niche differentiation in coprophagous flies is poorly studied. Sepsid flies (Diptera: Sepsidae) are a vital part of the European community of coprophages, with 6-7 widespread species of Sepsis often found co-occurring in the same pasture. To advance our ecological understanding of the mechanisms that enable species to coexist, we investigated the oviposition preferences and larval performance of 7 common species of Sepsis in the dung of different large domestic and wild mammals. Substrate preferences and subsequent performance of larvae in laboratory experiments did not vary greatly. All species did very well on cow dung, the most common substrate in Central Europe, but also on dung of horse and wild boar. In contrast, flies did not prefer or grow well in dung of red and roe deer, two of the most common wild vertebrates. Thus there were only minor differences among the species tested along the specialist-generalist (dung) gradient, indicating that differences in the choice of oviposition sites by the adults of the different fly species and larval performance do not constitute a major axis of ecological differentiation. Nevertheless, there was a positive correlation between substrate choice and larval performance indicating the preference of gravid females for particular oviposition sites is adaptive. We conclude that sepsids are common in Europe because they are well adapted to the dung of herbivorous livestock rather than wild animals. Nevertheless, specialization on particular types of dung does not define the niche of Sepsis dung flies and hence plays a minor role in mediating their species diversity.