Sixty-one of 2500 F344 rats were observed to have one to three enlarged and firm mammary glands. Pasteurella pneumotropica was isolated from all affected mammary glands. It was also routinely isolated from the feces of all cages and from the oral cavity of rats up to 6 weeks old, but not from the oral cavity of rats over 4 months of age. The organism was not isolated from clinically normal lactating mammary glands. Histopathological diagnosis of affected glands was chronic necrotizing mastitis.