Assam is one among the states with the highest Muslim population in India. According to the Population Census 2011, Muslims recorded 34% of the total population in the state, with a 3% increase in the last decade. The increasing Muslim population in the last several decades is mainly due to the immigration from East Pakistan and Bangladesh, which induced several conflicts regarding land resources with the local indigenous Hindu communities. Although the conflicts between the immigrant Muslims and indigenous Hindus are often reported in the media and argued in sociological studies, fieldwork-based studies on the livelihood of the immigrant communities in the locality are scarce. This study tries to reveal the differences in the agricultural practices and land-use patterns of immigrant and indigenous communities in the Brahmaputra floodplain and examines the sustainability of the multi-ethnic society of Assam. In Nagaon district of central Assam, it was found that the residential areas of immigrant and indigenous communities were clearly demarcated based on the ecological environment. Immigrant communities built their villages by the riverbank plain of the Brahmaputra, a region where river water inundates seasonally, while the indigenous community lives mostly by the central plain near towns. To adapt to excessive water and flooding, the immigrant people have developed intensive fish farming in the rainy season and Boro rice cultivation in the dry season by modifying the original flat topography and introducing artificial groundwater irrigation. Their adaptation techniques to the existing environment are entirely different from those of indigenous villages, and they are able to generate higher productivity per unit of land. These techniques resulted in considerable differences in the livelihood and income of both communities. However, economic linkages have also been found from the field observations suggesting weak ties between the communities.