Method 1603 describes a membrane filter (MF) procedure for the detection and enumeration of Escherichia coli bacteria in ambient waters and disinfected wastewaters. This method is a single-step modification of EPA Method 1103.1 ( mTEC). Unlike the mTEC media method, it does not require the transfer of the membrane filter to another substrate. The modified medium contains a chromogen (5-bromo-6-chloro-3-indolyl- Beta-D-glucuronide), which is catabolized to glucuronic acid and a red- or magenta-colored compound by E. coli that produces the enzyme Beta-D-glucuronidase. The apparatus and equipment, and sampling, filtration, and verification procedures for the modified mTEC method are identical to those of the original mTEC method. E. coli is a common inhabitant of the intestinal tract of warm-blooded animals, and its presence in water samples is an indication of fecal pollution and the possible presence of enteric pathogens. The E. coli test is recommended as a measure of ambient recreational fresh water quality. Epidemiological studies have led to the development of criteria which can be used to promulgate recreational water standards based on established relationships between health effects and water quality. The significance of finding E. coli in recreational fresh water samples is the direct relationship between the density of E. coli and the risk of gastrointestinal illness associated with swimming in the water. Method 1603 is approved for analysis of E. coli in fresh, estuarine, and marine waters. Method 1603 was submitted to interlaboratory validation in wastewater matrices. A summary of method performance results from this validation study are provided. A comprehensive evaluation of the study results is presented in the validation study report. Based on these validation study results, Method 1603 is proposed for approval for the analysis of E. coli in disinfected wastewater.