We cloned the genomic RNA of canine distemper virus (CDV) and determined the nucleotide sequence of the large (L) protein-coding gene. The L gene is 6573 nucleotides long and contains a single open reading frame coding for a polypeptide of 2161 amino acids (MW 246,354). The precise 5' end of the viral genome consists of a 38-nucleotide leader region. The CDV L protein shows over 77% amino acid similarity with its morbilliform relative measles virus (MV) with nearly 67% of their amino acids conserved. The sequence homology of 11 negative strand viruses L proteins is compared and relatedness was found in the following decreasing order: CDV, MV, Sendai virus, parainfluenza virus type 3, simian virus 5, parainfluenza virus type 2, mumps virus, Newcastle disease virus, respiratory syncytial virus, vesicular stomatitis virus, rabies virus. The consensus sequence of proposed functional domains involved in L gene catalytic activities was well conserved in the CDV L protein.