<I>Moraxella catarrhalis</I> is a gram-negative diplococcus which until recently was thought to be a harmless commensal. Increasing awareness has established the pathogenic nature of this organism and it is now recognised as a major cause of otitis media in children, exacerbations of chronic bronchitis in elderly patients and an occasional cause of invasive disease. <I>M. catarrhalis</I> is spread nosocomially especially in respiratory wards containing elderly patients. This study evaluated four methods for typing nosocomially spread isolates:- immunoblotting with normal human serum (NHS), and three DNA fingerprinting methods. The most discriminatory method found was restriction endonuclease analysis (REA) using <I>Taq</I> I, although immunoblotting with NHS and pulsed-field gel electrophoresis (PFGE) using <I>Sma</I> I sub-divided isolates grouped together by the other methods. PFGE using <I>Not</I> I only confirmed groupings made by other methods. A study of <I>M. catarrhalis</I> and phenotypically similar organisms was performed using comparisons of partial 16S rDNA sequence. 16S rDNA of <I>M. catarrhalis</I> strains from disparate geographical locations was found to be extremely conserved <I>M. catarrhalis</I> 16S rDNA was very similar to that of other <I>Moraxella</I> species whilst <I>Moraxella</I> species were found to be generally distinct from the <I>Neisseria</I> and <I>Kingella</I> species studied. These results confirm <I>M. catarrhalis</I> as a genuine member of the <I>Moraxellae</I>.