As part of a detailed soil study examining the concentration, form and fate of As in Bangladesh soils used in rice production, pedons from five agroecological zones in major flood prone regions of the country were described and sampled for characterization of physical, chemical and mineralogical properties needed for classification at the family level in U. S. Soil Taxonomy. The study soils were formed in silty and clayey alluvium of either the Ganges or Meghna Rivers. The alluvium in western Bangladesh, derived from the Ganges River, was calcareous, and contained both calcite and dolomite, as well as an abundance of weatherable minerals, especially micas in the sand and silt fractions. Clays were of mixed assemblage (mica, smectite, kaolinite, chlorite) in most pedons, but the soil from southern Rajshahi District is probably smectitic, with wide cracks in a hexagonal pattern open at the surface during the dry season. Endoaquepts were found in the Ganges River alluvium with control sections ranging from coarse-silty to fine. Two pedons representing the eastern portion of the country were sampled in Meghna River alluvium, which is noncalcareous and derived from a different source area. These soils are silty in the control section, contain abundant micas as well as other weatherable minerals in the sand and silt fractions, and have a mixed assemblage of clay minerals. Both Fluvaquents and Endoaquepts were described in this alluvium. Morphology of the study soils indicates seasonal reduction. Features include Fe and Mn concentrations in ped interiors, along root channels and in pores, and depletions at vertical ped surfaces (gleyans). Oxidation-reduction cycles of the Fe in these soils are believed to impact the form and mobility of As in these soils.