A 6-kb EcoRI genomic DNA fragment of Coxiella burnetii, isolated from a recombinant bacteriophage λZapII library, allowed heterologous genetic complementation of Escherichia coli deleted for its dnaJ gene. The C. burnetti dnaJ gene was expressed in E. coli and identified by Western blot analysis using polyclonal antibodies raised against purified E. coli DnaJ protein. Deletion mapping and genetic complementation demonstrated that C. burnetti dnaJ is present on a 2-kb EcoRI-HindIII genomic DNA fragment, from which the nt sequence of the C. burnetti dnaJ gene was determined.