Arsenic (As), a class 1 carcinogen is widely distributed in the environment. Since rice (Oryza sativa) takes up As in the form of arsenite (As(III)) in paddy field through highly expressed Si transporters Lsi1 and Lsi2, it is able to accumulate higher As in the grains compared with other cereal crops. As a result, rice is a major dietary source of inorganic As for the populations consuming rice as the staple food. Therefore, it is a very important issue for human health to reduce the As accumulation in rice grains. To identify genes controlling As accumulation in rice, we first investigated the genotypic difference in shoot As accumulation in 134 core collection rice varieties. By using hydroponic solution containing 1 µM As(III), we found that shoot As concentration ranged from 11 to 41 mg kg^-1 dry weight depending on cultivars after exposure for 6 days. Furthermore, indica cultivars accumulated significantly lower As in the shoots than japonica cultivars. To explore the underlying mechanisms, we used one F₂ population and one chromosome segment substitution lines (CSSL) population to map the responsible genes. The F₂ population was derived from a cross between an indica cultivar (lower As concentration) and an japonica cultivar (higher As concentration). By using the F₂ population, one major quantitative trait locus (QTL) for shoot As accumulation was detected on chromosome 10. This QTL explained 20-30% of total As variation. A QTL located at the same region was also detected by using the CSSL population derived from Muha (indica cultivar, low As) and Koshihikari (japonica cultivar, high As). In addition, another QTL on chromosome 6 was also mapped for shoot As concentration in this CSSL population. Fine mapping of these QTL genes is being undertaken by using high generation populations in future.