Several unclassified strains were obtained during investigations of the aerobic pharyngeal flora of healthy white laboratory mice originating from four different colonies. Nineteen strains formed a group tentatively designated taxon 12. Comparative investigations including taxon 12, P. ureae sensu stricto. "P. urea" strain Ackerman 80-443D, P. pneumotropica sensu stricto, P. pneumotropica type Heyl, P. pneumotropica type Henriksen and "H. influenzae-murium" showed that taxon 12 was identical with "P. ureae" strain Ackerman 80-443D but different from P. ureae sensu stricto and other species belonging to the family Pasteurellaceae Pohl 1981. The G + C content of DNA from the suggested reference strain for taxon 12 was within the limits for Pasteurellaceae. DNA:DNA hybridizations linked taxon 12 to the genus Actinobacillus via the avian hemolytic Actinobacillus-like complex (27). The name Actinobacillus muris is proposed for taxon 12.