Many intimate associations between different species of ectomycorrhizal fungi are inferred on the basis of the consistent co-occurrence of their fruit bodies. Suillus bovinus and Gomphidius roseus, where the latter never occurs without the former, is one
example. This association was examined with PCR identification and light microscopy. S. bovinus and G. roseus were unambiguously
separated on the basis of RFLPs of the PCR-amplified ITS region of ribosomal DNA. Tuberculate mycorrhizas of Pinus sylvestris
sampled under fruit bodies of G. roseus and S. bovinus were investigated and the majority were identified as mixed associations
involving both G. roseus and S. bovinus. Tuberculate mycorrhizas, which macroscopically resemble the ones of Suillus species,
contained typical chlamydospores of G. roseus and they had haustoria where G. roseus hyphae penetrated the cortical root cells. Pine
seedlings collected near the fruit bodies of the two species were mainly colonised by S. bovinus. Mycelial rhizomorphs collected
under the fruit bodies of G. roseus were identified as S. bovinus, while both fungal species were present at the base of G. roseus fruit
bodies. The significance of these observations and the possibility that G. roseus acts as a parasite are discussed.