The avian paramyxovirus type 1 (APMV-1), or Newcastle disease virus (NDV), 
comprise a diverse group of viruses with a single-stranded, negative-sense RNA 
genome. Historically, two systems have been simultaneously used to classify NDV 
isolates into lineages or genotypes, generating confusion in the nomenclature 
and discrepancies in the assignment of genetic groups. In the present study we 
assessed the genetic diversity of the avian paramyxovirus type-1 (APMV-1) and 
propose a unified nomenclature and a classification system based on objective 
criteria to separate NDV into genotypes. Complete F gene sequences of class I (n 
= 110) and class II (n = 602) viruses were used for the phylogenetic 
reconstruction and to identify distinct taxonomic groups. The mean 
interpopulational evolutionary distance was estimated (10%) and set as the 
cutoff value to assign new genotypes. Results of our study revealed that class I 
viruses comprise a single genotype, while class II contains 15 genetic groups 
including 10 previously established (I-IX, and XI) and five new genotypes (X, 
XII, XIII, XIV and XV). Sub-genotypes were identified among class I and class II 
genotypes. Adoption of a unified nomenclature and of objective criteria to 
classify NDV isolates will facilitate studies on NDV epidemiology, evolution, 
disease control and diagnostics.