DNA-DNA hybridization studies were conducted on six Pasteurella haemolytica-like (PHL) organisms recovered from cases of swine enteritis. Chromosomal-enriched fractions of PHL organisms served as the source of DNA for Southern blots or as whole-chromosomal DNA probes. Under stringent hybridization conditions, chromosomal DNA probes of a prototype PHL (strain 6213A) organism distinguished other PHL organisms from Pasteurella haemolytica types A1 and T3, Pasteurella multiocida types A:1 and A:3, Escherichia coli, Pseudomonas aeruginosa, Actinobacillus pleuropneumoniae type 1, and Salmonella cholerasuis. The guanine-cytosine content of the DNA of three PHL strains was 41.2 to 42.8 mol % as calculated from the thermal denaturation midpoint temperatures. The PHL strains are Gram-negative, nonmotile, beta-hemolytic, pleomorphic, oxidase-positive, urease- and indole-negative, fermentative rods with the key characteristics of the species Pasteurella haemolytica. None of the PHL strains reacted with the type-specific antisera of P. haemolytica types 1 through 12 as tested by an agglutination procedure. These swine strains differed in their biochemical differentiation from P. haemolytica types A1 and T3 in that all produced acid from M-inositol and failed to grow on MacConkey agar. Acid production from trehalose and L-arabinose was variable with PHL strains. Leukotoxicity of PHL strains was evaluated by a colorimetric micro-titration assay. Sterile culture supernatants of three of five PHL strains were toxic to bovine neutrophils. Results of these studies suggest that the PHL organisms may belong to a new group of organisms under the genus Pasteurella.