Microbicidal compositions for aqueous systems are provided that include (a) a nitrogenous compound activated by an oxidant or an enzyme and (b) 1,4-bis(bromoacetoxy)-2-butene (BBAB). The components are present in a combined amount synergistically effective to control the growth of at least one microorganism. Methods for controlling the growth of microorganisms in aqueous systems with the compositions are also provided.