This thesis investigated the means of identifying sources of bacterial contamination in rural drinking water wells. Total coliform, fecal coliform, fecal streptococci and 'Clostridium perfringens' were monitored in a laboratory investigation. Indicator organisms survived for 6 months and were present in warm and cold blooded animals. 'C. perfringens' was isolated from most animals assessed. Transmission electron micrographs revealed that cells similar to those observed in cell cultures, and manure were present in well water. Dividing cells were also observed. The assemblage of indicator organisms proved to be reliable indicators of fecal contamination providing information on relative timing and source. In 1997, over 300 rural drinking water wells throughout Southern Ontario, and 148 wells in rural Zimbabwe, were tested to assess the effectiveness of these indicators in the field. In Ontario, all of the wells were tested in the spring, and re-sampled in late summer. Just under 50% of the wells exceeded the Ontario Drinking Water Objectives (DWO) in spring and summer. On both occasions approximately 20% of the wells tested positive for 'C. perfringens ', an animal indicator. Zimbabwe wells were sampled during the dry season, June 1997. Ninety-five percent of the wells had bacteria in excess of Ontario DWO, and 56% of the wells tested positive for 'C. perfringens '. 'Escherichia coli' that is resistant to nalidixic acid (NAR) was used as a biotracer to confirm the utility of the selected suite of indicator organisms and to assess sources. Thirty sites were investigated. At 4 of these sites the contamination source was present within the well structure, and 24 of 26 sites were successful in isolating at least one source of fecal contamination entering the well. In some cases, bacteria moved into well water very rapidly. Stratigraphy, soil type, geology, well construction and farming practises were compared in groups containing high and low risk wells. High risk wells were shallower; shallower to bedrock and to the water table and were located most often in clay or clay loam soil. High risk wells were frequently located on top of older limestone deposits. Impermeable layers such as the rock shale or hardpan may offer some protection against bacterial transport to groundwater.