Using a recently developed reverse genetics system, we have generated a recombinant Newcastle disease virus (NDV) vaccine in which the gene encoding the hemagglutinin-neuraminidase (HN) has been replaced by a hybrid HN gene consisting of the cytoplasmic domain, transmembrane region, and stalk region of HN of NDV, and the immunogenic globular domain of HN of avian paramyxovirus type 4 (APMV4). The objective was to generate a chimeric live vaccine that induces a protective immune response against NDV by eliciting neutralizing antibodies against the fusion (F) protein, but which can be differentiated from wild-type NDV on the basis of different antibodies elicited by their HN proteins. Pathogenicity tests in day-old chickens showed that the recombinant was non-virulent (intracerebral pathogenicity index [ICPI]=0.00). A vaccination-challenge experiment in 4-week-old specific pathogen free chickens demonstrated that the recombinant was completely safe and was able to protect chickens from challenge with a lethal dose of virulent NDV. By using a secreted form of HN produced in Pichia pastoris, a test was developed that allowed serological differentiation between animals vaccinated with the recombinant vaccine and animals infected with NDV. These results demonstrate that genetically modified marker vaccines can be generated from small RNA viruses that lack non-essential genes.