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We studied mycorrhizal symbioses of Cephalanthera falcata (Orchidaceae) in a population that included giant individuals, some of which were over 50 cm in height and had more than 10 shoots, as well as in two nearby habitats without giant individuals. For C. falcata adults in the habitat with giant individuals, a fungal lineage in the ectomycorrhiza‐forming clade of Sebacinales was detected as the dominant mycobiont. Furthermore, higher 13C enrichment, which is an indicator of mycoheterotrophy, was found in these C. falcata individuals compared to those in the two nearby habitats. These results indicate that the growth of giant C. falcata individuals may be a result of the increased ability of Sebacinales fungi to transfer carbon to the orchids. Furthermore, we attempted to discern the effect of mycobiont taxa on seedling development in the three habitats. Contrary to our expectations, Sebacinales fungi were not found to significantly impact seedling growth as this orchid was predominantly colonized by Thelephoraceae fungi at the seedling stage. These results suggest that seedling growth is mostly enhanced by symbiosis with Thelephoraceae fungi in this orchid, and that the growth of giant individuals due to colonization by Sebacinales fungi may occur subsequently. [ABSTRACT FROM AUTHOR]