During routine quality control of laboratory rodents, short gram-negative rods with satellite growth adjacent to a Staphylococcus strain were isolated from rats. They proved to be members of the family Pasteurellaceae. On the basis of their dependence on V factor they were classified as Haemophilus sp. Systematic investigations in our laboratory rat colony revealed a high prevalence of these bacteria. They were isolated from 75 of 446 rats (16.8%) which were monitored by culture during a 2-year investigation. Most strains were isolated from the lungs and the trachea; some were cultured from the nasal cavity and the female genital tract. Antibodies to these bacteria were detected in sera from 385 of 829 rats (46.5%) by using an indirect immunofluorescence test. The majority of culturally and serologically positive animals came from three separate holding areas; they all came from the same breeder. Investigation of rats immediately on receipt from the breeder showed that they were culturally and serologically positive for Haemophilus sp. Histological examination of rats which were monoinfected with Haemophilus sp. showed a mild inflammatory cell infiltration in the lungs and a light diffuse hyperemia. In the physiological and biochemical investigations of 53 isolates, all strains had an identical biochemical profile. On the basis of the 35 criteria examined, a definite classification is not possible. These Haemophilus bacteria are probably members of a hitherto unknown species.