The resistance of avian infectious bronchitis virus (IBV) to several chemical and physical treatments was studied. Ten strains, including four Japanese strains, were used. 1. All strains were sensitive to heating at 56 degrees C for 15 minutes; although two of them, KH and Massachusetts-41, were resistant to heating at 45 degrees C for 90 minutes. 2. All strains were resistant to pH 3.0 and most of the strains were sensitive to pH 11.0. 3. All strains were completely inactivated by chloroform and sodium deoxycholate and all except Beaudette-42 and Connaught were relatively stable to ether. 4. All strains rapidly lost their infectivities upon ultraviolet irradiation. 5. Trypsin did not affect the infectivity of any strain. 6. From these results, the ten strains were classified into three groups based on their stabilities to exposure to heating at 45 degrees C for 90 minutes and to ether.