This document specifies a method for the enumeration of culturable microorganisms in water by counting the colonies on a low-nutrient agar culture medium after incubation at 22 °C for 7 d. The method is intended to measure the operational efficiency of the treatment process of public drinking water supplies, including the water in distribution systems and containers. The method is particularly suitable to monitor water for human consumption which is low in nutrients and is distributed in temperatures below 20 °C. The method can be applied to all types of water, including pool and spa waters. NOTE 1 The low-nutrient agar in use in this document usually gives higher colony counts from water samples than nutrient-rich formulations of culture media typically used for enumeration of culturable microorganisms. NOTE 2 The method is also applicable for waters of very low nutrient content such as de-ionised, distilled or reverse osmosis waters. NOTE 3 This document describes the use of R2A medium. There are other formulations available, e.g. R3A medium that might be suitable for certain applications but go beyond the scope of this document.