The role of arbuscular mycorrhizal fungus (AMF) on biotransformation of arsenic (As) in the rhizosphere of sunflower (Helianthus annuus L.) was studied in a rhizobag system with As-contaminated soil. The treatments consisted of a combination of two levels of AMF (Glomus aggregatum) inoculation (-AM and +AM) under sterile and non-sterile soil conditions (S and NS). Each treatment was designated as -AM/S, -AM/NS, +AM/S and +AM/NS. Sunflower seedlings were cultured in the rhizobag for 6 weeks. Rates of root AMF colonization in +AM treatments were approximately 40% and 0% in -AM/S treatment. The AMF inoculation reduced As toxicity symptoms and improved plant growth. Shoot As, but not the root As, concentrations were reduced by AMF inoculation. Shoot and root P concentrations increased in +AM treatments regardless of soil sterilization. Analyses of water soluble (WS) As in the soils at the end of the cultivation indicated that the amount of WS-arsenite (AsIII) was higher in the central compartment compared with the outer compartment, while WS-arsenate (AsV) was dominant in the outer compartment. Dimethylarsinic acid (DMAA) was detected only in +AM treatments and the concentration was higher in the +AM/NS treatment compared to the +AM/S. Trimethylarsine oxide (TMAO) was also detected in measurable amounts in the NS condition and AMF inoculation increased the amount of TMAO, especially in the central compartment. No methylated As species was detected in the -AM/S treatment. From these results, it is clear that the mycorrhizal roots colonized by Glomus aggregatum are primarily involved in DMAA formation. Aside from these mycorrhizal roots, the indigenous soil microorganisms in the mycorrhizosphere are responsible for promoting the transformation of DMAA into TMAO. These As biomethylation processes would favor the detoxification of As, especially at the interface of plant roots and rhizosphere soil. © 2007 Japanese Society of Soil Science and Plant Nutrition.