Internal segments of the gyrA and polA genes involved in DNA replication of Mycobacterium tuberculosis and rnhA of M. smegmatis, have been amplified by the polymerase chain reaction (PCR) using degenerate oligodeoxyribonucleotide primers based on conserved sequences. The deduced amino acid sequences were 54-66% homologous to the corresponding segments of their Escherichia coli counterparts. This method provides a useful means of cloning genes encoding DNA replication enzymes of mycobacteria.