Water samples from the wastewater treatment system of a bleach kraft mill and from the river that supplies the mill were plated on six different media and culturable isolates were screened for substrate utilization patterns, taxonomic characters, plasmid content, and resistance to ampicillin, streptomycin, kanamycin, tetracycline, naldixic acid, mercury, nickel, copper, cobalt, cadmium, and zinc. A cluster analysis of the substrate utilization profiles and taxonomic characters revealed thatPseudomonas,Acinetobacter, andAcidovoraxspp. were common among the culturable isolates from the river, whileAncylobacter aquaticus,Klebsiellaspp., and an unknown group of pleomorphic Gram-negative methylotrophs were common among the culturable isolates from the mill treatment system. Of isolates from the settling pond and aerated lagoon, 78 and 64% carried plasmids, while only 56% of isolates from the river carried plasmids. Plasmids were significantly associated with resistance to cadmium but not with any other resistance characters. Large numbers of plasmid-carryingA.aquaticusstrains and pleomorphic methylotrophs accounted for high plasmid incidence levels in the mill treatment system, and the ability to dechlorinate simple aliphatic substrates was found in these two groups as well as in onePseudomonasstrain.Key words: pulp and paper, wastewater treatment, chlorinated organics, culturable heterotrophs, methylotrophs, plasmids.