AIM The present research work was carried out to study the patho-epidemiological aspects of Genotype-XIII Newcastle disease virus (NDV) infection in commercial layer in and around Anand, Gujarat. As the outbreaks have reported in vaccinated flocks, it was felt necessary to study the disease with respect to its changing pathogenicity and relevant aspects. MATERIALS AND METHODS The study comprised of patho-epidemiology of Newcastle disease (ND) by information collected from different layer farms suffering from the disease in relation to incidence pattern and mortality, duration of mortality, susceptible age, and loss due to production performance. Clinical signs were recorded based on observations. During post-mortem, gross lesions were also recorded. For histopathological examination visceral organs according to lesions were collected in 10% formalin and processed slide stained by hematoxylin and eosin for microscopic examination. Cultivation of virus was done in embryonated specific pathogen-free (SPF) eggs of 9-11 days and isolation of virus was done for haemagglutination (HA) and haemagglutination inhibition (HI) test and to identify pathotype of virus by intracerebral pathogenicity index (ICPI) test to determine the virulence of virus. The Genotype-XIII NDV was confirmed by F gene sequence and whole genome sequence. RESULTS During the study mortality due to ND was recorded in 13 layer flocks in spite of routine vaccination, which usually contain Genotype-II strain of virus. The mortality was observed as high as above 50% with an average of 21.21%. The susceptible age for disease was found to be 6-14 weeks. The duration of mortality observed was 23 days. The disease resulted in a significant reduction in body weight, feed intake and drop in egg production. Majority of the outbreaks appeared during extremely hot months of April to June. Greenish diarrhoea was frequently seen in birds that survived early in infection. Mortality continued for 2-3 weeks and reduced with appearance of torticollis. Gross lesions were characterized by multifocal to diffuse hemorrhages around proventricular glands, necrotic (diphtheritic) haemorrhagic ulcers throughout the intestine, disseminated multiple foci of necrosis and pin-point hemorrhages in the spleen parenchyma. The microscopic lesions include focal to diffuse hemorrhages, diffuse infiltration of mononuclear cells, necrosis, and degeneration in visceral organs. All the 13 farm samples (n=13) resulted in death of all the embryos following incubation up to 72 h post-inoculation. All the 13 allantois fluids from field samples along with F and R2B vaccine sample were found positive for HA activity, which was further confirmed by HI using known NDV serum. The values of ICPI were 2.0 which were indicative of velogenic nature of the field NDV strain. CONCLUSION The study indicated that presently available live and attenuated vaccines which include Genotype-II NDV have failed in protecting the flocks against Genotype-XIII and resulted in outbreaks with mortality above 50%. ICPI score of 2.0 confirmed that the present outbreaks were due to Genotype-XIII NDV, which is velogenic in nature.