The complete nucleotide sequence of the PB2 gene of influenza virus A/Chile/1/83 (H1 N1) is presented. Sequence comparison between A/Chile PB2 protein and the known PB2 sequences of the influenza strains A/WSN/33 (H1 N1), A/PR/8/34 (H1 N1), A/NT/60/68 (H3 N2), A/Kiev/59/79 (H1 N1), A/FPV/Rostock/34 (H7 N1), and B/Ann Arbor/1/66 indicates extensive amino acid homology for the influenza A virus PB2 proteins. Small clusters of basic amino acids are conserved in all PB2 proteins including the influenza B PB2 protein which has only 39% sequence homology overall to the PB2 polypeptides of type A influenza viruses. The evolutionary rate of 5.7 x 10(-3) nucleotide substitutions per site per year and 0.25% amino acid changes per year between the A/Chile/1/83 and A/NT/60/68 PB2 appears to be higher than that calculated earlier for A/NT, A/PR/8 and A/WSN. An unusually high degree of sequence change between A/Chile/1/83 and A/Kiev/59/79 PB2 polymerase was revealed and this is discussed in terms of its probable origin.