The complete nucleotide sequence of the large (L) genome segment of Bunyamwera virus has been determined from overlapping cDNA clones. The segment is 6875 nucleotides long and has a base composition of 29.8V% A, 17.9% C, 15.4% G, and 36.9% U. Eighteen of the terminal 19 nucleotides at the 3' and 5' ends are complementary. In the viral- complementary (+ sense) RNA there is a single long open reading frame (ORF) from AUG at bases 51-53 to a UAG stop codon at bases 6765-6767; this ORF encodes a polypeptide of 2238 amino acids (MW 259,000), corrorresponding to the L protein which has been mapped to the L RNA segment by analysis of reassortants of Bunyamwera, Batai, and Maguari viruses. The aminoterminal 46 amino acids of the L protein show strong homology (63% identity) with the amino-termini of ORFs predicted from limited sequence analysis of the L segments of La Crosse and snow snowshoe hare bunyaviruses. Comparison with the polymerase proteins encoded by other negative-strand viruses showed weak homology with pad of the influenza virus PB1 protein, but no homology was detected with the other influenza virus polymerase proteins nor with the L proteins of arenaviruses, paramyxoviruses, and rhabdoviruses. At the 5' end of genomic (- sense) RNA there is an AUG-initiated ORF potentially encoding a protein of 14,700; the significance of this ORF is unknown at present.