Eight strains of avian paramyxovirus type 1 (PMV-1) isolated in Finland during the last 3 decades were studied with reverse transcriptase polymerase chain reaction (RT-PCR) and subsequent sequence analysis of the region of 208 nucleotides covering the fusion (F) protein cleavage site. Both genetic and antigenic heterogeneity of the strains was significant. Direct epidemiological links between strains isolated during successive outbreaks, and also between strains isolated from wild fauna and from poultry or captive birds, were seen in this study. These results also support the previously published view that wild waterfowl serve as a reservoir for the apathogenic or low-pathogenic strains, allowing them to evolve further into pathogenic strains.