A rapid coliphage analysis technique for enumerating coliphages in natural waters has been evaluated by water quality laboratories located throughout the United States. Correlations were established between coliphages and coliforms in natural water systems. These correlations were highly significant. This relationship can thus be used to determine the number of fecal or total coliforms present in natural water samples based on an enumeration of coliphages. With this method, coliphages in natural water systems (containing greater than or equal to six coliphages per 100 ml) can be enumerated within 6 h.