The present invention comprises a method for controlling (e.g. inhibiting) or killing microorganisms in an aqueous environment. The method includes the addition of an effective amount of monochlorourea or modified monochlorourea to an aqueous solution. This aqueous solution may be a cooling water system, a recreational water system, a water treatment facility, or any circulating water system (i.e. a papermaking facility). A method of producing monochlorourea or modified monochlorourea is also disclosed.