Abstract: The physiological state of introduced Flavobacterium strain P25 cells was determined in starvation cultures, in bulk soil, and in the rhizosphere of wheat using direct viable counts (DVC; based on cell elongation after use of nalidixic acid and substrate addition, resulting in a potential activity measurement) and the redox dye 5-cyano-2, 3-ditolyl tetrazolium chloride (CTC; based on respiration without substrate additions, resulting in an in situ activity measurement). Both methods clearly demonstrated that the metabolic activity of Flavobacterium P25 cells decreased during starvation, followed by increased activity after amendment with substrate. This confirmed the applicability of DVC and CTC methods to Flavobacterium P25. Both DVC and CTC methods showed that the percentage of active cells in an introduced Flavobacterium P25 population in rhizosphere soil was lower than that in bulk soil in the first 1–2 weeks after planting wheat seedlings. After two weeks, the percentage of metabolically active cells in the P25 population in rhizosphere soil was higher than in bulk soil. Since different aspects of cellular physiology are measured when applying DVC and CTC, the impact of variations in environmental factors on the metabolic state of introduced strains may be monitored closely by these methods.