Alcaligenes faecalis suppressed the growth of 11 strains of fungal plant pathogens in vitro. When it was cultivated on a synthetic medium containing (NH4)(2)SO4 as the sole nitrogen source, NH2OH, NO2- and NO3- were produced, indicating that heterotrophic nitrification was occurring. The suppressive effect of A. faecalis on plant pathogens was due to its NH2OH produced.