We have cloned and sequenced two overlapping fragments of chromosomal DNA from Actinobacillus actinomycetemcomitans. The nucleotide sequence contained two open reading frames. The deduced amino acid sequences of the two open reading frames showed significant homology with the heat shock proteins hsp70 and hsp40 of other organisms respectively. The upstream open reading frame consisted of 1902 bp, corresponding to 634-amino-acid residues. The CAA codon for glutamines was frequently seen in hsp70, i.e., in 30 of 32 glutamines(93.8%). The spacing region between the two open reading frames was unusually long compared with other prokaryotic organisms. A number of unique and distinguishing features of the sequences in the hsp70 family were verified, and it was found that a particular spacing sequence between the hsp70 and hsp40 gene loci can be used to identify A. actinomycetemcomitans from the periodontal pocket.