We preliminarily investigated the correlation between productivity and the diversity of free-living nitrogen-fixing bacteria on the rhizoplane of local rice varieties, including Siam Unus, Siam Adil and Siam Pandak, all tolerable to acid-sulfate soil in South Kalimantan, Indonesia. The rhizobacteria of some unidentified weeds of native-grown in the acidic paddocks were also searched. Subsequently, sphingomonads, including Sphingomonas spp. and Sphingobium spp., are found as the most dominant rhizobacteria in local rice and also in weeds that are adapted to the strongly acidic wet soil. Other unique rhizobacteria are Alcaligenes spp. that powerfully neutralized their cultured media. To understand how the local variety of paddy rice can tolerate acid-sulfate soils with very low soil pH (2.5-3.5), studies of these acid-tolerant, root-associating, and often nitrogen-fixing, rhizospherous bacteria are likely to be important key factors.