Tetramethyldithiooxamide (TMDTOA) is a stable and effective inhibitor of metal corrosion. The antimicrobial effects of TMDTOA and its metal compounds were investigated with regard to their use as biocides for water treatment. Growth of a variety of strains of bacteria and yeasts was completely inhibited at 400 μmol l−1 TMDTOA or its metal complexes. At 100 mg l−1 there was a 99.99% reduction in the number of viable micro-organisms; this activity persisted for 1-3 months. TMDTOA can be produced cheaply at 98% purity by a novel method, representing an alternative cost-effective water treatment agent combining corrosion-inhibiting and biocide characteristics.