Results of a previously described presence–absence (P–A) test were compared with those of the membrane filter (MF) technique for detecting fecal coliforms, coliforms, and anaerogenic coliforms in municipal drinking water samples. The P–A test allowed more frequent isolation of these organisms and, when supplemented by simple confirmatory tests, could also indicate the presence of fecal streptococci, green fluorescent pseudomonads, Aeromonas species, and Clostridium perfringens. In the absence of coliform detection, the occurrence of these other bacterial groups could give warning of unusual fecal or organic pollution.A comparison of coliform and non-coliform isolates from MF and P–A tests indicated that similar organisms were detected by both methods. The finding of false-positive "sheen" colonies was attributable to the presence of ethyl alcohol in the MF Endo medium. Although organisms giving rise to such colonies were non-coliforms, their presence was suggestive of inadequate water treatment.