This report describes the use of a bacterial 'fingerprint' technique to determine if treated effluent from infiltration beds was the source of coliform bacteria in a nearby well used for drinking water. Phage typing of bacteria is a method which has been used in hospitals to trace the source of infections, since each source usually has a characteristic phage type. Fecal coliform bacteria isolated from local wastewater infiltration basins, a nearby septic tank, and the contaminated well were phage typed. The typing indicated that there was a 17% similarity between the septic tank and the contaminated well, and a 27% similarity was found between the septic tank and the infiltration bed isolates. Finally there was a 52% similarity between the percolation bed isolates and the well isolates. Additional aspects of the subject are discussed.