The present invention provides methods and formulations for reducing or inhibiting increase in the concentration of microbes in a water-based fluid. The methods and formulations of the present invention use an oxazolidine compound and a hydroxymethyl-substituted phosphorus compound selected from the group consisting of tetrakis(hydroxymethyl)phosphonium salts, C1-C3 alkyl- and alkenyltris(hydroxymethyl)phosphonium salts and tris(hydroxymethyl)phosphine. The methods and formulations of the present invention can be useful in treating water contaminated with aerobic or anaerobic bacteria in oilfield and other industrial applications.
