A physical genome map of the Moraxella catarrhalis type strain (ATCC 25238) has been constructed using pulsed field gel electrophoresis. Macrorestriction analyses of the genome of M. catarrhalis were performed by digestion with the restriction enzymes Sinai, NotI, and RsrII, which cleave the single circular chromosome into 9, 10, and 6 fragments, respectively. The chromosomal fragments generated by pulsed field gel electrophoresis were convened to a linkage map utilizing a combination of partial digestions, and cross-hybridizations. Moraxella catarrhalis, like a number of other respiratory pathogens, has a relatively small genome estimated at 1750 kilobase pairs or about 40% of the size of the Escherichia coli genome. The locations of the four ribosomal RNA operons (rrnLS) were determined by Southern hybridization and by digestion with I-CeuI endonuclease. A number of genes involved in virulence have been placed onto the physical map by Southern hybridization including those encoding the predominant outer- membrane proteins and the chromosomal gene encoding β-lactamase.