We have cloned and sequenced the complete genome of a strain of poliovirus type 3 (23127) isolated during an outbreak of poliomyelitis in Finland. The genome is 7435 nucleotides long excluding the 3' poly(A) stretch and is 95.5% homologous at the amino acid level to the previously sequenced type 3 strain, P3/Leon/37. The most striking feature of the presented sequence is the extent of amino acid substitutions relative to P3/Leon/37 and other type 3 strains in areas of known antigenic importance. The major antigenic determinant for virus neutralization (site 1), located at residues 89 to 100 of VP1, has three amino acid substitutions and there are six substitutions in site 3, a composite site made up of sequences from VP1 and VP3. The variation in these regions probably accounts for the observed unusual antigenicity and may explain why the virus was able to spread in a well-vaccinated community. Sequence comparisons imply that the virus is not derived from the currently used live attenuated vaccine.