Application of the tritiated-thymidine method for measurement of the in situ cell formation rate of rhizosphere bacteria was investigated. The growth of bacteria was observed in a sterilized soil-plant system. Viable or freeze-killed sterilized sugar beet seeds were coated with a strain of a fluorescent Pseudomonas and grown for 2-4 days in pots with sterilized soil. The pots were incubated with tritiated thymidine, the DNA extracted, and the amount of ³H in DNA measured, and quantified as number of bacteria formed per unit time. On the basis of the incorporation rate of tritiated thymidine and plate counts, specific growth rates were calculated. By use of the tritiated-thymidine method, a growth rate of 4.9 × 10⁴ cells h^-1 cm root^-1 was obtained (generation time 106 h). Growth rates might be underestimated if thymidine is present in high concentrations in the soil or rhizosphere. Bacteria in the rhizosphere were responsible for 70% of the total bacterial growth in the pots. No significant ³H-labelling of plant root DNA was observed.