Drinking water in England and Wales exhibits high compliance with quality regulations. Severn Trent Water (STW) rarely experiences quality failures, yet approximately half of those that do occur are for bacteriological parameters. There were 218 bacteriological non-compliances across the STW region between January 2008 and December 2011. Thirty nine percent of these were from water treatment works and service reservoirs. Coliforms were the most commonly isolated indicator organisms. Two thirds of failures had no cause identified: of these most had residual free chlorines below 0.3 mg l−1; few were from samples with high water temperature; surface water and groundwater supplies had high proportions of failures, with blended supplies exhibiting lower incidence. This research calls for a more holistic approach to managing bacteriological water quality in distribution systems.