These are the soils of aquic condition with seasonal inundation for a few months during the monsoon season. Four representative pedons from the Lower Ganges delta area were studied for their morphological and clay mineralogical characteristics. The parent materials of the soils were calcareous Gangetic alluvium which have been decalcified over time. Being located in the lower part of the delta the soils are heavy-textured with the weighted average clay content ranging from 52 to 70% up to a depth of one meter. The soils have developed enormous redoximorphic features including redox concentration in the middle zone and a reduced matrix in the lower zone of the profiles. The soils were characterized at the family level of US soil taxonomy that will be useful in their intensive management. Smectite was the dominant mineral with weathered mica, and chlorite, along with small quantities of kaolinite and mixed layer minerals. Smectite mineral was thought to be inherited from the parent material and does not appear to be stable under the existing pedochemical environment wherefrom lime has been washed away. The smectite mineral was found to be an iron-rich beidellite type with an estimated chemical formula of (Si6.4 Al1.20) (Al2.41 Fe1.02 Mg0.57) O 20 (OH)4.0 X1.20.) The occurrence of considerable quantity of smectite has impact on the physical behavior as well as management of these soils. There is every possibility of fixation of applied potassium by the smectite mineral. However, there is considerable quantity of weathered mica in the soils from which potassium will be released. Layer silicate clay is the dominant fraction in the bulk mineralogy of soils. Since these seasonally flooded hydromorphic soils of Bangladesh are intensively used for growing paddy rice their mineralogy, morphology, classification and nutrient dynamics should be meticulously recorded for maintaining sustainablity in soil health as well as crop production.