We examined the nitrogenase reductase (nifH) genes of endophytic diazotrophic bacteria expressed in field-grown sweet potatoes (Ipomoea batatas L.) by reverse transcription (RT)-PCR. Gene fragments corresponding to nifH were amplified from mRNA obtained from the stems and storage roots of field-grown sweet potatoes several months after planting. Sequence analysis revealed that these clones were homologous to the nifH sequences of Bradyrhizobium, Pelomonas, and Bacillus sp. in the DNA database. Investigation of the nifH genes amplified from the genomic DNA extracted from these sweet potatoes also showed high similarity to various α-proteobacteria including Bradyrhizobium, β-proteobacteria, and cyanobacteria. These results suggest that bradyrhizobia colonize and express nifH genes not only in the root nodules of leguminous plants but also in sweet potatoes as diazotrophic endophytes.