The genetic diversity of the hypervariable region I of S1 gene (HVR I) of infectious bronchitis (IB) vaccine strains H120, Ma5 and 4/91 was compared to that of 26 infectious bronchitis virus (IBV) strains isolated from the field in Guangxi province of China during the years 1985-2008, and the field isolates were classified into five major genotypes. Monovalent antisera against three vaccine strains and seven field isolates of different genotypes were prepared by immunizing rabbits with mineral oil adjuvant preparations containing viruses propagated in chicken embryos. Virus neutralization (VN) tests were performed in tracheal organ cultures (TOCs) using these 10 strains with the antisera, and a one-way VN test was then used to compare the relationship of 10 monovalent antisera to the other 19 field isolates. As a result, seven different serotypes were classified based on the results of VN tests with the 26 isolates plus the three vaccine strains. We found that different serotypes were prevalent during different time periods, that more new serotypes have been prevalent in more recent years, and the prevalence of the original dominant serotype has been in constant decline since 2004. In addition, the concordance rate of the 26 field isolates between the S1 genotypes and serotypes was 57.7%.