Environmental contamination of arsenic (As) and its accumulation in rice (Oryza sativa L.) is of serious human health concern. In planta speciation of As is an important tool to understand As metabolism in plants. In the present study, we investigated root to shoot As translocation and speciation in rice exposed to inorganic and methylated As. Arsenate (As . Moreover, in shoots 78% MA III and 71% As III were present as weakly bound species which is alarming, as MA III has been found to be more cytotoxic than As III for human and it could also be an important factor inducing straighthead (spikelet sterility disorder) in rice.Arsenic (As) is ubiquitously present, considered as a non-essential metalloid for plants and animals, and poses serious health hazards to humans. High levels of As in soil and water have been reported around the world through geothermal, mining and industrial activities, agricultural applications and contaminated ground water 1,2 . Ground water of many countries, particularly in the Indian subcontinent, are naturally highly enriched with As and posing toxicity through drinking water and food chain contamination 3,4 . Agricultural fields are the net sink for thousands of tons of As being transferred each year through contaminated irrigation water in these areas 5,6 . The typical As concentration in soil solution of paddy fields varies from 0.01 to 3 μ M, however, as high as 33 μ M As has been reported in a paddy field irrigated with As-laden ground water 7,8 . Further, a significant yield loss has been reported in the crops grown in As contaminated areas 8 . Thus, a threat to the sustainability of food production has been recognized as the other side of the As calamity 8,9 . Understanding the mechanism of As toxicity in plants is crucial to find a sustainable solution to the problem. Arsenic exists in various chemical forms in the environment, which differ considerably in plant uptake, mobility and toxicity. Thus, speciation of As in various plant parts is an important tool to understand the in planta transformation and metabolism of various As species.Inorganic