Real-time PCR is a new and highly sensitive method for the quantification of microbial organisms in environmental samples. This work was conducted to evaluate real-time PCR with SybrGreen (SG) detection as quantification method for Desulfotomaculum lineage 1 organisms in samples of rice field soil. The method was optimized in several parameters like SG concentration. These allowed quantitative PCR with different primer combinations yielding PCR products with lengths up to 1066 bp and with sensitivities of 10(2) targets for all assays. The detection limit in environmental DNA extracts (rice bulk soil and rice roots) was 10(6) targets per gram dry weight according to the dilution of the DNA extracts necessary to overcome PCR inhibition of humic substances. A verification, that the fluorescence increase was due to specific PCR products, was done by agarose gel electrophoresis since melting curve analysis of the PCR products did not show a distinct peak in the first derivative, when the environmental DNA extracts were used in PCR. Amplification with a primer combination specific for Desulfotomaculum lineage 1 organisms showed an abundance of this group of approximately 2% and 0.5% of the eubacterial 16S rDNA targets in rice bulk soil and rice root samples, respectively. Approximately half of this number was obtained in both habitats with a PCR assay specific for a Desulfotomaculum sequence cluster obtained previously from rice field soil.