Protection of public health requires routine monitoring of environmental waters for fecal contamination, especially waters serving as drinking supplies. Municipalities typically rely on assays of indicator microorganisms to detect fecal contamination. Ideally, indicators and pathogens share a common origin. However, the indicators should have higher numbers, survive longer, and be easier to detect and quantify in the laboratory than pathogens. Coliform bacteria are the standard microbial indicator currently used in the United States. Enteric viruses are smaller and survive longer than coliform bacteria, therefore they are inadequate indicators of enteric viruses. Enteric bacteriophages, viruses infecting bacteria, may be a more reliable indicator for human viral pathogens. However, there are several taxonomic groups of enteric bacteriophages, all of which may not be good virus indicators. This project examines the ecology of coliphages, bacteriophages infecting Escherichia coli, in a ground water aquifer which serves as a municipal water supply.