In the field isolate, 91-58, of feline herpesvirus type 1 (FHV-1), one of the major immunogenic proteins was found to have different molecular masses of 75 and 130 kDa from those in the other field isolates (Maeda et al., J Vet Med Sci 57, 147-150, 1995). Immunoblot analysis using monoclonal antibodies (MAbs) indicated that the protein is glycoprotein C (gC). The gC gene of 91-58 was amplified by polymerase chain reaction (PCR) and shown to have an inserted fragment of approximately 160 base pairs (bp). Restriction endonuclease analysis of the PCR product with various restriction enzymes was carried out, indicating that the insertion located within 262 bp between Eco RV and DraI sites. Nucleotide sequence analysis indicated that the inserted fragment was 156 bp encoding 52 amino acids and composed repeat sequences. Next, five recent isolates were also examined by immunoblot analysis using anti-FHV-1 cat serum or MAbs. The result showed that one isolate, 98-064, also had the gC with different molecular weights. PCR and nucleotide sequence analyses indicated that 98-064 had an inserted sequence of 78 bp at the corresponding region identified in the gC gene of 91-58, although the inserted sequence was different from that of 91-58. These results indicated that some of FHV-1 isolates had the genetic rearrangements in the gC gene and detection of such mutations would be useful for differentiation among FHV-1 field isolates.