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The optimal incubation conditions for the direct viable count method with nalidixic acid were determined. They do not differ from those proposed in the literature for a laboratory strain and a mixed bacterial population isolated from drinking water. The direct viable count method with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) was performed under in situ conditions. The bacteria were incubated with CTC at concentration of 1 mM for 4-6 h at the temperature of the water in the pipes and without the addition of an exogenous substrate. The results obtained for a laboratory strain using the two direct count methods were similar. However, for a mixed bacterial population, the counts were always higher with the CTC method than with the nalidixic acid method.