The tropical pasture grass Brachiaria humidiola (Rendle) Schweick releases nitrification inhibitory compoundsfrom its roots, a phenomenon termed ‘biological nitrification inhibition’ (BNI). We investigated the influence ofroot exudates of B. humidicola on nitrification, major soil microorganisms and plant growth promoting microorganismsusing two contrasting soil types, Andosol and Cambisol. The addition of root exudates (containingBNI activity that is expressed in Allylthiourea unit (ATU) was standardized in a bioassay against a syntheticinhibitor of nitrification, allylthiourea, and their function in soil was compared to inhibition caused by the syntheticnitrification inhibitor dicyandiamide. At 30 and 40 ATU g)1soil, root exudates inhibited nitrification by95% in fresh Cambisol after 60 days. Nitrification was also similarly inhibited in rhizosphere soils of Cambisolwhere B. humidicola was grown for 6 months. Root exudates did not inhibit other soil microorganisms, includinggram-negative bacteria, total cultivable bacteria and fluorescent pseudomonads. Root exudates, when addedto pure cultures of Nitrosomonas europaea, inhibited their growth, but did not inhibit the growth of severalplant growth promoting microorganisms, Azospirillum lipoferum, Rhizobium leguminosarum and Azotobacterchroococcum. Our results indicate that the nitrification inhibitors released by B. humidicola rootsinhibited nitrifying bacteria, but did not negatively affect other major soil microorganisms and the effectivenessof the inhibitory effect varied with soil type.