A rapid, simple, and accurate counterimmunoelectrophoresis (CIE) technique was developed and compared with the indirect hemagglutination and rapid plate agglutination techniques for serotyping cultures of Pasteurella haemolytica. The CIE test had 100% correlation with the conventional indirect hemagglutination test and, after serum absorption, correctly identified cultures representing the 12 established serotypes and 49 field isolates of P. haemolytica with reasonable rapidity. Cross-reactions were observed in the CIE and rapid plate agglutination tests but not in the indirect hemagglutination test with antisera prepared from the 12 established serotypes. These cross-reactions were eliminated from the CIE test but not from the rapid plate agglutination test by absorption of antisera with cells which possessed the cross-reacting antigens. Avian isolates of P. haemolytica did not type with antisera to the 12 established serotypes by any of the methods. Both homologous and heterologous reactions were observed with these strains in the rapid plate agglutination and CIE tests with antisera prepared from six selected cultures. These results support the previous finding that the taxonomic relationship of these avian strains to P. haemolytica is questionable.