Disinfected and non-disinfected samples have been used to determine the accuracy of the ISO procedure (ISO 9308-1) for detection of E. coli in drinking water. Samples were analysed using the ISO procedure at both 36 and 44°C and using the defined substrate technology method Colilert-18®/Quanti-Tray® (Colilert-18). Utilizing the confirmation procedure described in ISO 9308-1, large numbers of false positive E. coli results were obtained using the ISO primary isolation procedure at 36°C. However, when glucuronidase production was used as the confirmation procedure, the ‘confirmed’ count of E. coli was lowest with ISO 9308-1 performed at 36°C. When TTC medium was incubated at 36°C confirmation using production of indole at 44°C resulted in 29% more ‘E. coli’ being recovered than when confirmation was performed using production of glucuronidase. When 44°C was used as the primary isolation temperature the difference between the number of ‘confirmed’ E. coli identified using the two confirmation procedures was less than 1% and was not significant. Identification of isolates which ‘confirmed’ when using production of indole at 44°C as the test method but °which failed to produce β-D-glucuronidase, showed that the majority of these isolates were Klebsiella oxytoca.