Four field isolates (S4, S10, S15, and S17) of Haemophilus paragallinarum were recovered from chickens affected with infectious coryza in widely separated regions of Japan. Their hemagglutinating (HA) activity and immunological properties were compared with those of strain 221 of serovar A/1 and strains Modesto and S1 of serovar C/2. When treated with potassium thiocyanate or hyaluronidase, all the isolates showed HA activity against formaldehyde-fixed chicken erythrocytes but not against fresh chicken erythrocytes. In the hemagglutination-inhibition (HI) test, the isolates cross-reacted with strains Modesto and S1 but not with strain 221. The immunological properties of these isolates, as determined by cross-protection tests, were similar to those of strain S1 and, to a lesser degree, strain Modesto, but not to strain 221. Our results indicated that the four field isolates belong to serovar C/2 and that the HI test is a suitable method for serotyping H. paragallinarum.