The authors report the results of characterization studies of three strains of Newcastle disease virus (NDV) (two isolated from pigeons and one from chickens). The plaque cloning of the viruses, showed that each NDV strain consists of different clones of genetically mixed viral populations. The pigeon NDV isolates were classified as lentogenic using mean death time (MDT) determination; while the intracerebral pathogenicity index (ICPI) was the same as the velogenic NDV strain.