The boundary of the study in this organisation involved laboratory experiments on the facets of the nature of coliform (Klebsiella) bacteria in parts of the Sydney water system which supplies 3.3 × 106 people. After September 1976 they appeared increasingly throughout the 18 000 km of reticulation system. Non-thermotolerant Klebsiella spp were persistent despite swabbing and flushing, their survival being enhanced by deposits in mains and turbidity greater than one unit in 75 % of 813 water samples. A multiple approach (biochemistry, genetics, typing, antibiotic and metallic sensitivity, and plasmid transfer) was adopted using 848 isolates in 1980 from 38 sampling runs. While the species persisted from 1980 to 1982, their designation of oxytoca remained unclear despite usage of 57 biochemical tests and G + C of 56.3. Cultures from both dates were bacteriocin typed, half being untypable as were the majority of 73 environmental cultures collected between Traralgon and Townsville in adjoining states. Antibiotic sensitivity testing by diffusion and agar dilution procedures demonstrated multi-resistance to twenty antibiotics that exceeded or equalled results reported from other habitats. Heavy metal resistance was considerably elevated. These two sets of markers are plasmid home and were more actively transferred at 37 than 20° C by environmental Klebsiella to E. coli K12. Because of the inability of the typing method, the pattern or origin of the contamination could not be found. The clinical significance of this activity at 37° C would require another study elsewhere because these bacteria are ingested.