Coliphage presence in ground water is an indication of fecal contamination. Method 1602 is a performance based method for enumerating male-specific (F+) and somatic coliphage in ground water and other waters. Laboratories are permitted to modify or omit any steps or procedure, with the exception of the coliphage stock enumeration procedure (Section 11.3), provided that all performance requirements set forth in the validated method are met. The laboratory may not omit any quality control analyses. This single agar layer procedure requires the addition of host bacteria, magnesium chloride, and double strength molten agar medium to the sample, followed by pouring the total volume of the mixture into plates. All plates from a single sample are examined for plaque formation (zones of bacterial host lawn clearing). The quantity of coliphage in a sample is expressed as plaque forming units (PFU) / 100 mL. This method is for use in the Environmental Protection Agency's (EPA's) data gathering and monitoring programs under the Safe Drinking Water Act and the Clean Water Act.