Tritirachium egenum sp. nov., a biotrophic mycosymbiont, was found growing in association with a Penicillium rugulosum. This new species was unable to grow in axenic culture on traditional semi-synthetic culture media unless the growth medium was supplemented with a fraction of the culture filtrate of its host. The hot water extract of Alternaria alternata, containing the 'growth factor' of several contact mycosymbionts (biotrophic contact mycoparasites) also supported the growth of T. egenum. Signs, particularly the functional equivalence of this extract and ferrichrome on the T. egenum growth, suggested that this growth factor, referred to in the literature as mycotrophein, could actually be a hydroxamate-type siderophore. Moreover, it was shown that this siderophore-dependent organism had a deficient metabolism requiring, in addition, an exogenous source of thiamine or a precursor molecule of thiamine. Among mycosymbionts of fungi, the nutrient acquisition strategy of T. egenum is new, because it did not live in close association with the cytoplasm of its host. Indeed, it is neither a haustorial mycosymbiont nor a contact mycosymbiont, the two groups into which the biotrophic mycosymbionts of fungi are classified. Moreover, an interfungal association based on the utilization of siderophores has not yet been reported