Field experiments were conducted in northern Thailand to evaluate the effect of five herbicides and three watering schedules on soybean yields and Nz fixation. The herbicides sethoxydim, alachlor, fluazifop butyl and metolachlor when added at recommended rates for weed control did not result in detrimental effects on seed yields or benefits of N2 fixation. Paraquat additions significantly reduced the amount of N2 fixed as measured by ~SN dilution methods. Inoculation of the soil with a mixed strain inoculum of Bradyrhizobium japonicum increased seed yields and measurements of Nz fixation. The application of water to soybeans only when water stress symptoms such as wilting were observed resulted in drastic reductions in both yield and N 2 fixation of four cultivars of soybeans. Reducing the water application from weekly to biweekly did not seriously reduce yields or N2 fixation benefits.