Abstract Under laboratory conditions coronaviruses were shown to have a high frequency of recombination. In The Netherlands, vaccination against infectious bronchitis virus (IBV) is performed with vaccines that contain several life-attenuated virus strains. These highly effective vaccines may create ideal conditions for recombination, and could therefore be dangerous in the long term. This paper addresses the question of the frequency of recombination of avian coronavirus IBV in the field. A method was sought to detect and quantify recombination from sequence data. Nucleotide sequences of eight IBV isolates in a region of the genome suspected to contain recombination, were aligned and compared. Phylogenetic trees were constructed for different sections of this region. Differences in topology between these trees were observed, suggesting that in three out of eight strains in vivo RNA recombinant had occurred.