A sequence 375 nucleotides in length, which included the region encoding the cleavage activation site and signal peptide of the fusion protein gene, was determined for 174 isolates of Newcastle disease virus (avian paramyxovirus type 1). These were compared with the sequences of 164 isolates published on GenBank, and the resulting alignment was analysed phylogenetically using maximum likelihood. The results are presented as unrooted phylogenetic trees. Briefly, the isolates divided into six broadly distinct groups (lineages 1 to 6). Lineages 3 and 4 were further subdivided into four sublineages (a to d) and lineage 5 into five lineages (a to e). Considerable genetic heterogeneity was detected within avian paramyxoviruses type 1, which appears to be influenced by host, time and geographical origin. It is concluded that by using this dataset it will be possible to type future virus isolates rapidly on the basis of their nucleotide sequence and make inferences about their origins.