Aerobic and facultatively anaerobic bacteria isolated from false-positive, presumptive, total coliform, most-probable-number tests of Chesapeake Bay oyster, water, and sediment samples were characterized and then classified by numerical taxonomy. A total of 538 bacterial strains clustered into 17 phena, the predominant groups of which were Enterobacteriaceae (including Escherichia coli), Aeromonas spp., and Bacillus spp. Bacillus spp. were recovered most frequently from sediment samples. Gas-producing strains which were not members of the Enterobacteriaceae were not isolated during this study. However, disproportionately large numbers of atypical and anaerogenic lactose-fermenting strains were encountered. We concluded that no single, specific bacterial group can be identified as being responsible for the false-positive reaction in the presumptive coliform test. Instead, the false-positive reaction is a result of complex interactions among various genera, representing predominantly bacteria other than coliforms. The most-probable-number (MPN) method for the detection of coliforms is a classical test which has been used since the turn of the century to determine the hygienic quality of drinking water and foods, as well as aquatic environments (2, 54). The association of cholera, typhoid, and, later, hepatitis, with ingestion of shellfish taken from water contaminated with domestic sewage resulted in the adoption of the MPN test as an index of shellfish quality (3, 18,