The capacity of arbuscular mycorrhizal (AM) fungi to take up and translocate radiocaesium (Cs) to their host has been shown using the root-organ culture (ROC) system. However, the absence of photosynthetic tissues, lack of a normal root hormonal balance and incomplete source–sink relationships may bias the bidirectional transfer of elements at the symbiotic interface and complicate transport studies. Accordingly, we developed a novel culture system [i.e. the Arbuscular Mycorrhizal-Plant (AM-P) in vitro culture system], where AM fungi and an autotrophic host plant develop under strict in vitro conditions. With this system, we unambiguously demonstrated the capacity of AM fungi to transport Cs. The extraradical fungal hyphae took up 21.0% of the initial supply of 134Cs. Translocation to the plant represented 83.6% of the 134Cs taken up. Distribution of 134Cs in the host plant was 89.8% in the mycorrhizal roots and 10.2% in the shoot. These results confirm that AM fungi can take up, translocate and accumulate Cs. They further demonstrate unambiguously and for the first time that Cs can be transferred from AM fungi to host tissues. These results suggest a potential involvement of AM fungi in Cs biogeochemical cycle and in plant Cs accumulation.