The determination of the presence or absence of B. coli in a drinking water is universally regarded as the estimation of most value in determining its safety or as grounds for its condemnation. Valuable as this estimation is it is desirable that, if possible, it should be reinforced and confirmed by the determination of the presence or absence of other organisms. In particular it would be of extreme value and importance if the isolation of other selected organisms could be used to throw light upon two points in regard to which evidence is either lacking or indefinite at the present time. These two matters are the question as to whether ascertained contamination is of human or animal origin and as to the probable date of any contamination shown to be present. The B. coli enumeration does throw considerable light upon the latter, but none upon the former point.