• In contrast to mycoheterotrophs that associate with ectomycorrhizal and saprotrophic fungi, we know little about the ecophysiology of arbuscular mycorrhizal mycoheterotrophs. Here, we identify the mycorrhizal fungi of two unrelated mycoheterotrophs and one putative partial mycoheterotroph that form arbuscular mycorrhizas, and analyse their carbon (C) and nitrogen (N) isotope signatures.

• We used molecular methods to identify the mycorrhizal fungi of Dictyostega orobanchoides, Burmannia capitata (Burmanniaceae) and Voyria aphylla (Gentianaceae). Their C and N sources were investigated by analysing their stable isotope natural abundances (δ¹³C and δ¹⁵N). In addition, four putative partially mycoheterotrophic Burmannia species were grown ex situ.

• We found that both mycoheterotrophs and a green Burmannia species are associated with nonoverlapping Glomeromycota fungi. The investigated mycoheterotrophs are significantly more enriched in ¹³C than co-occurring autotrophic plants but lack significant ¹⁵N enrichment. The green Burmannia species is not significantly enriched in ¹³C and ¹⁵N compared with surrounding plants and can grow fully autotrophically under controlled conditions.

• Our results suggest that mycoheterotrophic Burmanniaceae and Gentianaceae are able to exploit arbuscular mycorrhizal fungi. Green relatives of mycoheterotrophic Burmanniaceae from high-light grassland sites also associate with arbuscular mycorrhizal fungi but we found no evidence that they receive detectable amounts of C from fungi.