APMVs have been isolated from many different avian hosts [17] . APMV-1 is the only well-characterized serotype, because of the high morbidity, mortality, and economic loss caused by highly virulent strains. NDV isolates vary greatly in their pathogenicity for chickens, ranging from no apparent disease to severe respiratory and neurological disease causing 100% mortality [18] . NDV strains are categorized into three main pathotypes: lentogenic (avirulent), mesogenic (moderately virulent), and velogenic (virulent), based on their pathogenicity in chickens [19] . In contrast, the disease potential of APMV-2 to -9 is not well known because many of these viruses were isolated from birds dying in quarantine, hunter killed, trapped wild birds, apparently healthy poultry or exotic birds [20] . APMV-2 and -3 have been reported to cause significant disease in poultry, whereas the pathogenic potential of APMV-4 to -9 is generally unknown. In general, APMV-2 strains have been isolated from chickens, turkeys and wild birds across the globe and have been found to cause mild respiratory disease, decreases in egg production, and infertility [21] [22] [23] . APMV-3 strains have been isolated from wild and domestic birds and their infections have been associated with encephalitis and high mortality in caged birds [24] . APMV-5 strains have only been isolated from budgerigars (Melopsittacus undulatus) and cause depression, dyspnoea, diarrhea, torticollis, and acute fatal enteritis in immature budgerigars, leading to very high mortality [25] . Infections from APMV-4, -8, and -9 appear to be restricted to ducks and geese. APMV-6 and -7 infections in turkeys cause drops in egg production and induce respiratory disease. There are no reports of isolation of APMV-5, -8 and -9 from poultry [2] . But recent serosurveillance of commercial poultry farms in the U.S. indicated the possible prevalence of all APMV serotypes excluding APMV-5 in chickens [26] . The pathogenicity of APMV-2 and APMV-3 has been studied in experimentally infected chickens [27, 28] . However, replication, pathogenicity, and neurovirulence of APMV serotypes 2 through 9 have not been comprehensively studied. Therefore, we characterized in vitro replication of APMVs (growth kinetics and cytopatic effect in chicken fibroblast cells) and their in vivo replication and tropisms by infecting prototype strains of each serotype in two different ages of chickens (1-day-old and 2-week-old chickens) and 3-week-old ducks. Specifically, neurotropism of APMVs was evaluated in primary chicken neuronal cells and brain tissue of 1-day-old chicks.