ABSTRACTAlthough more than 60 million individuals rely upon the absence of microbial pathogens in their marginally‐treated or untreated ground‐water supplies, an analysis of reported waterborne disease outbreaks for the period 1946‐1970 shows that contaminated ground‐water supplies were responsible for over 50 percent of the outbreaks. Completed ground‐water studies indicate: (1) coliforms and fecal coliforms are present in a significant percentage of improperly located or inadequately protected private supplies, and (2) the apparent absence of coliforms due to the insensitivity of currently available bacteriological methods does not preclude pathogen occurrences. Excessive bacterial populations, normally not encountered in finished water, can suppress coliform detection. For this reason, it is essential that improved bacterial detection methods be developed and other criteria for untreated ground water be explored by comprehensive field investigations and laboratory analysis of ground‐water supplies for a variety of bacterial