The MPN test of coli-form group is used as a biological indicator, but, as the bacteria designated in the coli-form group are not always derived from feces, the number of coli-form group bacteria obtained by the standard method results in a much higher count than expected. On the contrary, enterococci which are mostly derived from feces are thought to be more useful as pollution indicators. In the present work, the availability of enterococci as a biological indicator was studied. No enterococcus was detected in water which was assumed not to be pollution by feces of human or animal origin. Enterococci did not grow in river water under any condition, and died in river water much faster than the coli-form group. As the enterococci numbered as much as one-half to one twentieth of coli-form group in human feces, the author standardized the numbers of enterococci for the grades of water environment as one tenth of the coli-form group, and applied the system to field work. From the results in the field, it can be concluded that enterococci are more useful as a biological indicator of water pollution than the coli-form group.