Potential response regulator gene fragments from the genome of Branhamella (Moraxella) catarrhalis were isolated by PCR using degenerate oligonucleotide primers. DNA sequence analysis of several cloned PCR products with similar restriction endonuclease analysis (REA) patterns revealed that the cloned gene fragment had significant homology to members of the OMPR sub-family of response regulator genes, including 61% identity with the phoB gene of Haemophilus influenzae. The derived amino acid sequence showed greatest similarity to the PhoB response regulator protein of Pseudomonas aeruginosa. In the last 10^15 years, Branhamella (Moraxella) catarrhalis has emerged as one of the prominent human pathogens associated with infections of the ear and the lower respiratory tract where it is the major cause of exacerbations of chronic bronchitis in adults [2,4]. Little is known about the virulence mechanisms of this pathogen [4]. In this study we targeted the regulatory elements of virulence rather than the virulence genes themselves. Two-component signal transduction systems play a central role in the coordinate regulation of virulence in many bacteria and these systems are comprised of a sensor and a response regulator protein [3]. The aim of this study was to identify response regulator genes of B. catarrhalis.B. catarrhalis clinical isolates K69, K70, K106 and K116 were recovered from sputum samples collected from patients at the Sir Charles Gairdner Hospital, Perth, Western Australia. Isolates were cultured on Mueller Hinton agar containing 0.5% yeast extract. DNA was extracted from these isolates and from The GenBank accession numbers for the N-terminal homologous region of the proposed phoB nucleotide sequence of Branhamella (Moraxella) catarrhalis are U80193 (clinical isolate K69) and U80194 (clinical isolate K70).