Moraxella catarrhalis is a Gram-negative diplococcus, morphologically and biochemically related to the Neisseria. It is usually found in the commensal flora of both children and adults, and most commonly colonises the mucous membranes of the nasopharyngeal tract. However, M. catarrhalis can also cause severe systemic infections, such as pneumonia, meningitis, otitis media and endocarditis. The microbe is fast becoming an important opportunisitic pathogen, and produces a number of virulent proteins on host invasion. Two ubiquitously expressed proteins on the cell surface of M. catarrhalishave been shown to confer protection in a mouse model against the microbe;both resemble adhesins from other pathogenic bacteria. Designated UspA1 and UspA2, they share significant amino acid sequence similarity and are prime candidates for vaccine development owing to their immunogenic nature. Clinical samples taken from patients has revealed little diversity in the genes; UspA1 appears to be the most promising target.