question: Abstract The nucleocapsid protein of coronaviruses has been considered highly conserved, showing greater than 94% conservation within strains of a given species. We determined the nucleotide sequence of the N gene and the 3′ untranslated region (UTR) of eight naturally occurring strains of IBV which differed in pathogenicity and tissue tropism. In pairwise comparisons, the deduced amino acid sequences of N of five strains Vic S, N1/62, N9/74, N2/75, and V5/90 (group I) shared 92.3–98.8% identity. The three strains N1/88, Q3/88, and V18/91 (group II) shared 85.8–89.2% identity with each other, but only 60.0–63.3% identity with viruses of group I. Amino acid substitutions, deletions, and insertions occurred throughout the N protein and involved regions previously identified as being conserved. Despite the considerable variation observed between the two virus groups, all N proteins contained a high proportion of basic residues, 80% of which were conserved in position. In addition, all strains contained approximately 30 serine residues of which 10 were conserved, the majority occurring between positions 168 and 194. As for all other coronaviruses, the region between positions 92 and 103 was highly conserved. Hence, a large number of amino acid changes can be tolerated within the N protein without affecting its integrity or functioning. The 3′ UTR immediately downstream from the N gene was highly heterogeneous with extensive deletions occurring in the group II strains.