The microbiological quality of the five leading brands of Norwegian bottled still waters was investigated. All brands were free for the enteric indicator organisms and named pathogens whose absence is demanded in current quality directives. The relatively nutrient-poor agar R₂A revealed large heterogeneous bacterial populations which grew slowly, or not at all, on clinical media specified for use in substrate-utilization approaches to identification. The main approach used for identification was cultivation of microbes on R₂A, followed by amplification and partial sequencing of 16S rDNA genes. The identity of the heterotrophic plate count of the brands differed significantly to that found in many other similar studies with respect to the dominating species. The bacterial flora was dominated by beta- and alphaproteobacteria most of which were psychrotolerant. Several brands contained Sphingomonas and large populations of Methylobacterium species which have been associated with a variety of opportunistic infections of immunocompromised hosts. Analysis of the isolated strains' nutritional capabilities using the Biolog GN2® system, gave in most instances low positive scores, and strain identifications using the system were generally inconclusive. Measures of assimilable organic carbon in the water revealed that some brands contained levels higher than those which have been associated with biological stability and restricted or no growth of heterotrophs in distribution systems. The relationship between assimilable organic carbon and R₂A plate counts was significant and moderately positive for bottled waters. Assimilable organic carbon correlated strongly with the survival time of Escherichia coli when introduced into bottles as a contaminant. Those brands having high values (~100 μg/L) supported protracted survival, but not growth of E. coli, whereas E. coli quickly became nonculturable in brands with low values. Thus measures of assimilable organic carbon may have a particular value in predicting the survival of this and nutritionally similar species of hygienic relevance. Only small numbers of fungi were found. However, one isolate (Aureobasidium pullulans) has been associated with infections of humans.