Syncytium formation is a hallmark of the cytopathic effect (CPE) caused by many paramyxoviruses, including APMV-1, in cell culture [1] . To investigate syncytium formation by APMV serotypes 2-9 and to compare their CPE, Vero cells were infected with mesogenic APMV-1 strain BC or with representatives of the other APMV serotypes at a multiplicity of infection (MOI) of 0.1 PFU/cell, incubated for 48 h, and visualized directly by photomicroscopy (data not shown) and following immunostaining with rabbit antiserum to the respective N protein (Fig. 1 ). Exogenous protease was supplemented into the culture medium for replication of APMV-3, 8, and 9, based on our previous studies [4, 9, 10] . APMV-1 strain BC was known to be independent of protease supplementation. APMV-3 and -5 produced distinctive CPE with syncytium formation similar to those of APMV-1. In contrast, the rest of APMVs produced single cell infections leading to cell rounding and detachment of infected cells but a lack of evident syncytia. Similar results were observed in chicken embryo fibroblast DF1 cells (data not shown).