The putative viral transcriptase p90 in infectious bursal disease virus (IBDV) was shown to form an enzyme-guanylate intermediate which is indicative of guanylyl-transferase activity. The p90-nucleotide bond is most likely to be a phosphodiester linkage, as it resisted treatment with HCl and NH2OH but was sensitive to NaOH. This is in contrast to phosphoamide bonds formed by reovirus cores. Methyltransferase activity was also demonstrated in IBDV, and is closely associated with transcription, suggesting that p90 may be a multifunctional enzyme.