The present invention provides synergistic combinations of monochlorourea with other biocides for controlling microbial growth in aqueous systems. It also provides synergistic combinations of dimethyl monochlorourea with other biocides for controlling growth in aqueous systems. The synergistic combinations of monochlorourea and dimethyl monochlorourea with other biocides allows for the reduced use of total biocides to provide control of microbial growth in aqueous systems.